Add PKCE support to redmine_oauth_controller

[juvex-fi]

Add PKCE Support to Redmine OAuth Plugin

This pull request adds PKCE (Proof Key for Code Exchange) support to the redmine_oauth_controller.

Changes:

• Generate code_verifier and code_challenge for the OAuth flow.
• Include code_challenge and code_challenge_method in the authorization request.
• Use code_verifier during the token exchange to improve security.

Benefits:

• Enhances security by preventing authorization code interception.
• Complies with OAuth 2.0 PKCE standards.

Tested with the following providers:

• Custom

[picman]

Please fix Rubocop errors.

[picman]

Thank you!

[picman]

I can confirm that it works with Azure AD too.