Add OAuth2MetadataUrl API and provider interface

Expose OAuth2 metadata URL support in the public API: add OpenApiConstants.OAuth2MetadataUrl constant, introduce IOAuth2MetadataProvider with an OAuth2MetadataUrl getter (System.Uri?), and surface OAuth2MetadataUrl on OpenApiSecurityScheme (get/set) and OpenApiSecuritySchemeReference (get). This enables consumers to read/write the OAuth2 metadata URL for security schemes.

Author: mdaneri

src/Microsoft.OpenApi/PublicAPI.Unshipped.txt

@@ -1,7 +1,13 @@
 #nullable enable
+const Microsoft.OpenApi.OpenApiConstants.OAuth2MetadataUrl = "oauth2MetadataUrl" -> string!
 const Microsoft.OpenApi.OpenApiConstants.UnevaluatedPropertiesExtension = "x-jsonschema-unevaluatedProperties" -> string!
+Microsoft.OpenApi.IOAuth2MetadataProvider
+Microsoft.OpenApi.IOAuth2MetadataProvider.OAuth2MetadataUrl.get -> System.Uri?
 Microsoft.OpenApi.IOpenApiSchemaWithUnevaluatedProperties
 Microsoft.OpenApi.IOpenApiSchemaWithUnevaluatedProperties.UnevaluatedPropertiesSchema.get -> Microsoft.OpenApi.IOpenApiSchema?
 Microsoft.OpenApi.OpenApiSchema.UnevaluatedPropertiesSchema.get -> Microsoft.OpenApi.IOpenApiSchema?
 Microsoft.OpenApi.OpenApiSchema.UnevaluatedPropertiesSchema.set -> void
 Microsoft.OpenApi.OpenApiSchemaReference.UnevaluatedPropertiesSchema.get -> Microsoft.OpenApi.IOpenApiSchema?
+Microsoft.OpenApi.OpenApiSecurityScheme.OAuth2MetadataUrl.get -> System.Uri?
+Microsoft.OpenApi.OpenApiSecurityScheme.OAuth2MetadataUrl.set -> void
+Microsoft.OpenApi.OpenApiSecuritySchemeReference.OAuth2MetadataUrl.get -> System.Uri?