Implement jwks endpoint and add test for it.

This also let to some improvements/cleanups of the other tests and fixtures.

IdentityShroud.Core/Services/IRealmService.cs

@@ -8,4 +8,5 @@ public interface IRealmService
     Task<Realm?> FindBySlug(string slug, CancellationToken ct = default);
         
     Task<Result<RealmCreateResponse>> Create(RealmCreateRequest request, CancellationToken ct = default);
+    Task LoadActiveKeys(Realm realm);
 }