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

首先阅读 /server/auth/handlers/token.py，特别是现有的 authorization-code 处理，并将其与 issue 中链接的 MCP OAuth Grant Types 规范进行比较。确认预期的 client-credentials 行为，并找出 repository 中相关的测试或测试入口点。当 token handler 支持 client-credentials 和 authorization-code，且不要求 PKCE 交换时，即表示完成。