Add OIDC authentication flows tests for federation

[xek]

This adds support for testing all OIDC authentication methods:

• v3oidcpassword (Resource Owner Password Credentials)
• v3oidcclientcredentials (Client Credentials)
• v3oidcaccesstoken (Access Token Reuse)
• v3oidcauthcode (Authorization Code)

New features:

• Templates for each OIDC auth method configuration
• Helper script to obtain tokens from Keycloak
• Keycloak client setup for Service Accounts and Device Auth
• Comprehensive test suite with assertions
• Updated README with full documentation

Note: v3oidcdeviceauthz requires Python 3.10+ and is not
available in OSP18 which ships with Python 3.9.

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