Client Credentials In the token Handler

Is your feature request related to a problem? Please describe.
The MCP Authorization Spec 2.1.1 OAuth Grant Types mentions that both Authorization Code and Client Credentials should be handled.

Describe the solution you'd like
In /server/auth/handlers/token.py, we only currently support "authorization-code". It would be fantastic if we could support "client-credentials" as well.

https://github.com/modelcontextprotocol/python-sdk/blob/544176770b53e6a0ae8c413d3b6c5116421f67df/src/mcp/server/auth/handlers/token.py#L26-L47

Describe alternatives you've considered
I mean it's just more convenient, so I don't have to do the whole PKCE exchange thing if it's not necessary.

Additional context

Link to MCP spec: https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization#2-1-1-oauth-grant-types

/server/auth/handlers/token.py を読み始め、特に既存の authorization-code 処理を確認し、issue にリンクされている MCP OAuth Grant Types 仕様と比較してください。期待される client-credentials の動作を確認し、リポジトリ内の関連するテストまたはテストのエントリーポイントを特定してください。token handler が authorization-code に加えて client-credentials もサポートし、PKCE 交換を要求しない状態になれば完了です。