internal/oauthex: add dynamic client registration (#519)

This CL implements https://www.rfc-editor.org/rfc/rfc7591.html

See
https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/client/auth.ts
as a reference.

For #493

internal/oauthex/testdata/client-auth-meta.json

@@ -0,0 +1,8 @@
+{
+    "client_name": "My Test App",
+    "redirect_uris": [
+        "https://client.example.org/callback",
+        "https://client.example.org/callback2"
+    ],
+    "scope": "read write"
+}