API access with client credentials (core functionality) (#16817)

* First stab

* Delivery API client credentials + a little refactor to ensure unique client IDs

* Introduce user type

* Support user type in the Management API

* Clean up TODOs

* Update API user last login date when issuing a token

* Better error reporting for mismatched user types

* Do not allow password change or reset for API users

* Update OpenApi.json

* Revert change

* Remove obsolete comment

* Make applicable classes abstract or sealed

* Review changes

* Add endpoint for retrieving all user client IDs

src/Umbraco.Cms.Api.Common/DependencyInjection/UmbracoBuilderAuthExtensions.cs

@@ -54,10 +54,14 @@ public static class UmbracoBuilderAuthExtensions
                     .RequireProofKeyForCodeExchange()
                     .AllowRefreshTokenFlow();
 
+                // Enable the client credentials flow.
+                options.AllowClientCredentialsFlow();
+
                 // Register the ASP.NET Core host and configure for custom authentication endpoint.
                 options
                     .UseAspNetCore()
                     .EnableAuthorizationEndpointPassthrough()
+                    .EnableTokenEndpointPassthrough()
                     .EnableLogoutEndpointPassthrough();
 
                 // Enable reference tokens