feat: add initial access token support for OAuth 2.0 Dynamic Client Registration (RFC 7591)

- Add initial_access_token parameter to OAuthClientProvider constructor
- Implement multi-level fallback for token resolution:
  1. Explicit parameter (highest priority)
  2. Provider method (initial_access_token())
  3. Environment variable (OAUTH_INITIAL_ACCESS_TOKEN)
  4. No token (existing behavior)
- Add Authorization Bearer header to registration requests when token available
- Add comprehensive test coverage for all fallback scenarios
- Update documentation with usage examples and configuration details
- Maintain full backward compatibility with existing OAuth flows

This enables clients to register with protected OAuth endpoints that require
initial access tokens per RFC 7591 Dynamic Client Registration specification.

Author: andormarkus-alcd