feat: add OAuth2 protected resource metadata endpoint for RFC 9728 (#18643)

# Add OAuth2 Protected Resource Metadata Endpoint

This PR implements the OAuth2 Protected Resource Metadata endpoint according to RFC 9728. The endpoint is available at `/.well-known/oauth-protected-resource` and provides information about Coder as an OAuth2 protected resource.

Key changes:
- Added a new endpoint at `/.well-known/oauth-protected-resource` that returns metadata about Coder as an OAuth2 protected resource
- Created a new `OAuth2ProtectedResourceMetadata` struct in the SDK
- Added tests to verify the endpoint functionality
- Updated API documentation to include the new endpoint

The implementation currently returns basic metadata including the resource identifier and authorization server URL. The `scopes_supported` field is empty until a scope system based on RBAC permissions is implemented. The `bearer_methods_supported` field is omitted as Coder uses custom authentication methods rather than standard RFC 6750 bearer tokens.

A TODO has been added to implement RFC 6750 bearer token support in the future.

coderd/apidoc/docs.go

@@ -65,6 +65,26 @@ const docTemplate = `{
                 }
             }
         },
+        "/.well-known/oauth-protected-resource": {
+            "get": {
+                "produces": [
+                    "application/json"
+                ],
+                "tags": [
+                    "Enterprise"
+                ],
+                "summary": "OAuth2 protected resource metadata.",
+                "operationId": "oauth2-protected-resource-metadata",
+                "responses": {
+                    "200": {
+                        "description": "OK",
+                        "schema": {
+                            "$ref": "#/definitions/codersdk.OAuth2ProtectedResourceMetadata"
+                        }
+                    }
+                }
+            }
+        },
         "/appearance": {
             "get": {
                 "security": [
@@ -13450,6 +13470,32 @@ const docTemplate = `{
                 }
             }
         },
+        "codersdk.OAuth2ProtectedResourceMetadata": {
+            "type": "object",
+            "properties": {
+                "authorization_servers": {
+                    "type": "array",
+                    "items": {
+                        "type": "string"
+                    }
+                },
+                "bearer_methods_supported": {
+                    "type": "array",
+                    "items": {
+                        "type": "string"
+                    }
+                },
+                "resource": {
+                    "type": "string"
+                },
+                "scopes_supported": {
+                    "type": "array",
+                    "items": {
+                        "type": "string"
+                    }
+                }
+            }
+        },
         "codersdk.OAuth2ProviderApp": {
             "type": "object",
             "properties": {