pub async fn build_singleton(&self) -> anyhow::Result<ConnectionPool<DB>> {
        let singleton_builder = Self {
            database_url: self.database_url.clone(),
            max_size: 1,
            acquire_timeout: self.acquire_timeout,
            acquire_retries: self.acquire_retries,
            statement_timeout: self.statement_timeout,
            _db: PhantomData,
        };
        singleton_builder.build().await
    }
Builds a connection pool that has a single connection.
core/lib/db_connection/src/connection_pool.rs