feat: integrate proxy support for REST/gRPC/OIDC and add new OIDC authentication grant

[fer-marino]

This PR introduces comprehensive proxy support for both REST and gRPC communications, as well as for OIDC authentication.

Changes

Features

• Proxy Support: Introduced a new Proxy record to hold proxy configuration (host, port, scheme, and optional credentials).
• Global Configuration: Added a proxy(Proxy) method to the Config builder, allowing users to configure a proxy globally for all client operations.
• New Authentication Method: Added resourceOwnerPasswordCredentials to the Authentication interface, supporting the Resource Owner Password Credentials authorization grant.
• REST Transport Integration: Updated DefaultRestTransport to use the configured proxy with Apache HttpClient 5.
• gRPC Transport Integration: Added proxy support for gRPC communication in DefaultGrpcTransport.
• OIDC Proxy Support: Integrated proxy support into NimbusTokenProvider and OidcUtils, ensuring that OIDC metadata and token requests (fetching and refreshing) correctly respect the proxy settings.

Refactoring

• Improved OIDC Metadata Handling: Refactored OIDC metadata fetching to be more robust and better integrated with the transport layer.

Testing

• New Unit Tests: Added ProxyTest and updated AuthenticationTest to verify proxy integration for both REST requests and OIDC authentication flows (including Resource Owner Password grant).

Verification

• Ran existing and new unit tests to confirm that proxy settings are correctly passed to the underlying transports.
• Verified that OIDC token fetching works through a proxy by mocking the OIDC server responses.
• Confirmed that gRPC communication respects the proxy configuration.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

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[bevzzz]

Nice addition, thank you @fer-marino

[fer-marino]

added some more test units, including one for the proxy. I've also modified the pom, as in my setup the lombok annotation processor was not getting invoked during unit tests execution.

[bevzzz]

added some more test units, including one for the proxy

Thank you 👍

[fer-marino]

pushed another commit with your suggestions

[bevzzz]

@fer-marino please update your branch to the latest main state, there're some changes that should fix the failing tests in the pipeline.

+3 pending comments ⬆️

[fer-marino]

rebased

[bevzzz]

@fer-marino looks like the commit hashes got changed in the rebase and the history/diff in this PR now includes changes already present in the main branch. Could you please squash your changes in to a single commit and force-push the branch?

Something like:

git reset --soft origin/main
git commit -m "feat: add proxy support and ROPC authorization grant"
git push --force-with-lease

It'll make it easier for me to wrap up the review.