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

<!-- How would you explain this change to the user? If this
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/token_test.go

@@ -0,0 +1,48 @@
+package oauth21_test
+
+import (
+	"net/http"
+	"net/url"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/pomerium/pomerium/internal/oauth21"
+)
+
+func TestParseTokenRequest_BasicAuth(t *testing.T) {
+	form := url.Values{}
+	form.Set("grant_type", "authorization_code")
+	form.Set("code", "abc")
+	req, err := http.NewRequest(http.MethodPost, "/token", strings.NewReader(form.Encode()))
+	require.NoError(t, err)
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.SetBasicAuth("myclient", "secret")
+
+	tr, err := oauth21.ParseTokenRequest(req)
+	require.NoError(t, err)
+	require.NotNil(t, tr.ClientId)
+	require.Equal(t, "myclient", *tr.ClientId)
+	require.NotNil(t, tr.ClientSecret)
+	require.Equal(t, "secret", *tr.ClientSecret)
+}
+
+func TestParseTokenRequest_BasicAuthWithBodyOverride(t *testing.T) {
+	form := url.Values{}
+	form.Set("grant_type", "authorization_code")
+	form.Set("code", "abc")
+	form.Set("client_id", "bodyid")
+	form.Set("client_secret", "bodysecret")
+	req, err := http.NewRequest(http.MethodPost, "/token", strings.NewReader(form.Encode()))
+	require.NoError(t, err)
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.SetBasicAuth("basicid", "basicsecret")
+
+	tr, err := oauth21.ParseTokenRequest(req)
+	require.NoError(t, err)
+	require.NotNil(t, tr.ClientId)
+	require.Equal(t, "bodyid", *tr.ClientId) // body should win
+	require.NotNil(t, tr.ClientSecret)
+	require.Equal(t, "bodysecret", *tr.ClientSecret)
+}