mcp: client registration/token fixes (#5649)

## Summary

Fixes to MCP code registration and token requests. 

1. ease some requirements on fields that are RECOMMENDED 
2. fill in defaults
3. store both request and response in the client registration
4. check client secret in the /token request

## Related issues

- Fixes
https://linear.app/pomerium/issue/ENG-2462/mcp-ignore-unknown-grant-types-in-the-client-registration
- Fixes
https://linear.app/pomerium/issue/ENG-2461/mcp-support-client-secret-in-dynamic-client-registration
 
## User Explanation

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change doesn't create any user-facing changes, you can leave
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## Checklist

- [x] reference any related issues
- [x] updated unit tests
- [x] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [ ] ready for review

internal/oauth21/proto/authorization_request.proto

@@ -34,10 +34,10 @@ message AuthorizationRequest {
   // OPTIONAL. The scope of the access request as described by Section 1.4.1.
   repeated string scopes = 5;
 
-  // REQUIRED, assumes https://www.rfc-editor.org/rfc/rfc7636.html#section-4.1
-  string code_challenge = 6 [
-    (buf.validate.field).required = true,
-    (buf.validate.field).string   = {min_len: 43, max_len: 128}
+  // REQUIRED or RECOMMENDED, assumes https://www.rfc-editor.org/rfc/rfc7636.html#section-4.1
+  // subject to whether the client is public or confidential.
+  optional string code_challenge = 6 [
+    (buf.validate.field).string = {min_len: 43, max_len: 128}
   ];
 
   // OPTIONAL, defaults to plain if not present in the request. Code verifier