Add PKCE support (RFC 7636)

[grudloffev]

Description

This PR introduces full support for the OAuth2 PKCE (Proof Key for Code Exchange) flow to the python-keycloak package.

Highlights

• PKCE support in code

  • Implements code challenge and verifier handling in the authorization flow.
  • Adds PKCE utilities for code_verifier and code_challenge generation.
  • Updates async and sync methods in KeycloakOpenID to accept PKCE parameters.
  • Adds MIT license header to new modules.

• PKCE tests

  • Adds tests for PKCE flow.
  • Validates both sync and async PKCE flows.

• Documentation

  • Updates OpenID client documentation to explain PKCE usage and integration.

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Checklist:

• PKCE support implemented and tested
• Documentation updated
• All tests passing

[grudloffev]

fixes #635

[grudloffev]

hmm I think the failed test was a fluke, seems to be a transient connection issue

[grudloffev]

Merged main to trigger retest