Remove federated_credentials_flow

[chlowell]

Closes #2282 by moving the client_assertion implementation into ClientAssertionCredential

[azure-sdk]

API change check

API changes are not detected in this pull request.

[Copilot]

Pull Request Overview

This PR removes the federated_credentials_flow module and moves its client assertion logic into the ClientAssertionCredential implementation, while updating related code in the SDK.

• Removed the federated_credentials_flow module and its associated files.
• Integrated client assertion token retrieval into ClientAssertionCredential with updated request and error handling logic.
• Updated client_secret_credential to use the new deserialize API and improved error messaging.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sdk/identity/azure_identity/src/lib.rs	Removed federated_credentials_flow module import; added a new deserialize helper function with enhanced error context.
sdk/identity/azure_identity/src/federated_credentials_flow/response.rs	Removed as part of the module deprecation.
sdk/identity/azure_identity/src/federated_credentials_flow/mod.rs	Removed the entire federated_credentials_flow module implementation.
sdk/identity/azure_identity/src/credentials/client_assertion_credentials.rs	Migrated token retrieval logic into a new get_token_impl method; updated endpoint construction, error handling, and tests.
sdk/identity/azure_identity/src/client_secret_credential.rs	Updated usage of deserialize function and error message construction for consistency.

Comments suppressed due to low confidence (1)

sdk/identity/azure_identity/src/credentials/client_assertion_credentials.rs:86

• [nitpick] Consider renaming the 'endpoint' variable to 'token_endpoint' to more clearly indicate that it represents the URL for token acquisition.

let endpoint = options.authority_host()?.join(&format!("/{tenant_id}/oauth2/v2.0/token"))