Add x-oauth2-metadata-url serialization for OAuth2 schemes

When serializing OAuth2 security schemes, the OAuth2MetadataUrl is now written as 'x-oauth2-metadata-url' for OpenAPI 3.1 and later. Updated tests and public API documentation to reflect this change.

Author: mdaneri

src/Microsoft.OpenApi/Models/OpenApiSecurityScheme.cs

@@ -128,6 +128,10 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
                     {
                         writer.WriteProperty(OpenApiConstants.OAuth2MetadataUrl, OAuth2MetadataUrl?.ToString());
                     }
+                    else
+                    {
+                        writer.WriteProperty("x-oauth2-metadata-url", OAuth2MetadataUrl?.ToString());
+                    }
                     writer.WriteOptionalObject(OpenApiConstants.Flows, Flows, callback);
                     break;
                 case SecuritySchemeType.OpenIdConnect:

test/Microsoft.OpenApi.Tests/Models/OpenApiSecuritySchemeTests.cs

@@ -312,6 +312,7 @@ public async Task SerializeOAuthSecuritySchemeWithMetadataUrlAsV31JsonOmitsMetad
                                 {
                                     "type": "oauth2",
                                     "description": "description1",
+                                    "x-oauth2-metadata-url": "https://idp.example.com/.well-known/oauth-authorization-server",
                                     "flows": {
                                         "clientCredentials": {
                                             "tokenUrl": "https://idp.example.com/oauth/token",

test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt

@@ -288,6 +288,7 @@ namespace Microsoft.OpenApi
         Microsoft.OpenApi.OpenApiOAuthFlows? Flows { get; }
         Microsoft.OpenApi.ParameterLocation? In { get; }
         string? Name { get; }
+        System.Uri? OAuth2MetadataUrl { get; }
         System.Uri? OpenIdConnectUrl { get; }
         string? Scheme { get; }
         Microsoft.OpenApi.SecuritySchemeType? Type { get; }
@@ -538,6 +539,7 @@ namespace Microsoft.OpenApi
         public const string Null = "null";
         public const string Nullable = "nullable";
         public const string NullableExtension = "x-nullable";
+        public const string OAuth2MetadataUrl = "oauth2MetadataUrl";
         public const string OneOf = "oneOf";
         public const string OpenApi = "openapi";
         public const string OpenIdConnectUrl = "openIdConnectUrl";
@@ -1381,6 +1383,7 @@ namespace Microsoft.OpenApi
         public Microsoft.OpenApi.OpenApiOAuthFlows? Flows { get; set; }
         public Microsoft.OpenApi.ParameterLocation? In { get; set; }
         public string? Name { get; set; }
+        public System.Uri? OAuth2MetadataUrl { get; set; }
         public System.Uri? OpenIdConnectUrl { get; set; }
         public string? Scheme { get; set; }
         public Microsoft.OpenApi.SecuritySchemeType? Type { get; set; }
@@ -1400,6 +1403,7 @@ namespace Microsoft.OpenApi
         public Microsoft.OpenApi.OpenApiOAuthFlows? Flows { get; }
         public Microsoft.OpenApi.ParameterLocation? In { get; }
         public string? Name { get; }
+        public System.Uri? OAuth2MetadataUrl { get; }
         public System.Uri? OpenIdConnectUrl { get; }
         public string? Scheme { get; }
         public Microsoft.OpenApi.SecuritySchemeType? Type { get; }