clients should check PKCE support

[jba]

The client auth implementation for clients should check that the auth server implements PKCE, as described in https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/docs/specification/draft/basic/authorization.mdx?plain=1#L350-L356.

[RyoKusnadi]

Hi @jba, allow me to double confirming

1. In auth_meta.go, Use GetAuthServerMeta and check using CodeChallengeMethodsSupported
2. provide example in client_example_test.go

let me know if i missed something.
if already no issue, can assign this task to me? willing to raise the PR

[jba]

Yes, CodeChallengeMethodsSupported must be a non-empty slice.