<!-- Describe your changes in detail -->
add core changes for external authentication flow through unified_authentication_service

Currently, Only ClickToPay is supported through Unified Authentication Service. This PR adds support to perform external authentication through connectors like Netcetera, through Unified Authentication service.

1. Add authentication flow in UnifiedAuthenticationService trait
2. Refactor UnifiedAuthenticationService trait
3. Refactor call_unified_authentication_service_if_eligible in payments confirm operation to support external authentication
4. Add Authenticate flow for UnifiedAuthenticationService connector integration

Will take up the connector side changes in a separate PR
To implement this **search_filter** feature, you'll need to modify these architectural layers:


**1. Domain Models (Business Logic Types)**
   - `crates/hyperswitch_domain_models/src/router_flow_types/unified_authentication_service.rs`
   - `crates/hyperswitch_domain_models/src/router_request_types/unified_authentication_service.rs`
   - `crates/hyperswitch_domain_models/src/types.rs`

**2. Core Business Logic**
   - `crates/router/src/core/payments.rs`
   - `crates/router/src/core/payments/connector_integration_v2_impls.rs`
   - `crates/router/src/core/payments/flows.rs`
   - ... and 7 more files

**3. External Connector Integration**
   - `crates/hyperswitch_connectors/src/connectors/unified_authentication_service.rs`
   - `crates/hyperswitch_connectors/src/connectors/unified_authentication_service/transformers.rs`
   - `crates/hyperswitch_connectors/src/default_implementations.rs`

**4. Type Definitions**
   - `crates/hyperswitch_interfaces/src/types.rs`
   - `crates/router/src/types.rs`

**5. Other**
   - `crates/hyperswitch_interfaces/src/api.rs`
   - `crates/router/src/routes/payments.rs`

**Code Patterns Applied:**
- Define new data structures (7 changes)
- Implement new functions (16 changes)
- Extend existing types with new fields (266 changes)
- Add conditional logic for feature flags or filtering (7 changes)

**Detailed Identifier Changes:**


Commit ae39374c:
Files to modify:


**crates/hyperswitch_connectors/src/connectors/unified_authentication_service.rs**
  Modify existing code

**crates/hyperswitch_connectors/src/connectors/unified_authentication_service/transformers.rs**
  Modify existing code

**crates/hyperswitch_connectors/src/default_implementations.rs**
  Modify existing code

**crates/hyperswitch_domain_models/src/router_flow_types/unified_authentication_service.rs**
  Modify existing code

**crates/hyperswitch_domain_models/src/router_request_types/unified_authentication_service.rs**
  Modify existing code

**crates/hyperswitch_domain_models/src/types.rs**
  Modify existing code

**crates/hyperswitch_interfaces/src/api.rs**
  Modify existing code

**crates/hyperswitch_interfaces/src/types.rs**
  Modify existing code

**crates/router/src/core/payments.rs**
  Modify existing code

**crates/router/src/core/payments/connector_integration_v2_impls.rs**
  Modify existing code

**crates/router/src/core/payments/flows.rs**
  Modify existing code

**crates/router/src/core/payments/helpers.rs**
  Modify existing code

**crates/router/src/core/payments/operations.rs**
  Modify existing code

**crates/router/src/core/payments/operations/payment_confirm.rs**
  Modify existing code

**crates/router/src/core/unified_authentication_service.rs**
  Modify existing code

**crates/router/src/core/unified_authentication_service/transformers.rs**
  Modify existing code

**crates/router/src/core/unified_authentication_service/types.rs**
  Modify existing code

**crates/router/src/core/unified_authentication_service/utils.rs**
  Modify existing code

**crates/router/src/routes/payments.rs**
  Modify existing code

**crates/router/src/types.rs**
  Modify existing code
{"pr_number": 7063, "pr_title": "feat(router): add core changes for external authentication flow through unified_authentication_service", "merged_at": "2025-02-03T20:30:05Z", "intent": {"feature_type": "search_filter", "action": "add_feature", "entities": ["payment", "connector"]}, "hierarchy": {"domain": ["crates/hyperswitch_domain_models/src/router_flow_types/unified_authentication_service.rs", "crates/hyperswitch_domain_models/src/router_request_types/unified_authentication_service.rs", "crates/hyperswitch_domain_models/src/types.rs"], "core_business": ["crates/router/src/core/payments.rs", "crates/router/src/core/payments/connector_integration_v2_impls.rs", "crates/router/src/core/payments/flows.rs", "crates/router/src/core/payments/helpers.rs", "crates/router/src/core/payments/operations.rs", "crates/router/src/core/payments/operations/payment_confirm.rs", "crates/router/src/core/unified_authentication_service.rs", "crates/router/src/core/unified_authentication_service/transformers.rs", "crates/router/src/core/unified_authentication_service/types.rs", "crates/router/src/core/unified_authentication_service/utils.rs"], "connectors": ["crates/hyperswitch_connectors/src/connectors/unified_authentication_service.rs", "crates/hyperswitch_connectors/src/connectors/unified_authentication_service/transformers.rs", "crates/hyperswitch_connectors/src/default_implementations.rs"], "types": ["crates/hyperswitch_interfaces/src/types.rs", "crates/router/src/types.rs"], "other": ["crates/hyperswitch_interfaces/src/api.rs", "crates/router/src/routes/payments.rs"]}, "change_types": {"structs_added": 7, "functions_added": 16, "enums_added": 2, "fields_added": 266, "imports_added": 11, "conditionals_added": 7}, "has_identifier_data": true, "commits_matched": 1, "match_method": "pr_number"}