feat: Implement OAuth2 authentication for REST catalog

[lishuxu]

Add OAuth2 authentication support for the REST catalog, including:

• OAuth2AuthManager with static token and client_credentials grant flows

Also includes:

• Fix BuildHeaders in http_client.cc to use insert_or_assign so request-specific
  headers properly override defaults

TODO:
RefreshToken and ExchangeToken will be supported later

[wgtmac]

This is a comprehensive code review report generated by gemini-cli against PR #577. The review rigorously compares the C++ implementation with the RFC 6749 (OAuth2) spec and the current Iceberg Java behavior.

Overall, the structure and C++ style are excellent, but there are a few parity and robustness issues that need addressing.