AddApiAuthorization for IdentityServer should allow to override in memory signing credential store.

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

We are using the Microsoft.Extensions.DependencyInjection.IdentityServerBuilderConfigurationExtensions.AddApiAuthorization method and it registers and in memory implementation of ISigningCredentialStore and IValidationKeysStore. The existing code will configure the ConfigureSigningCredentials and always call the LoadKey method in Microsoft.AspNetCore.ApiAuthorization.IdentityServer.ConfigureSigningCredentials. That means that you need to configure the IdentityServer:Key in your environment, even if you end up not needing it because you use your own implementation of ISigningCredentialStore and IValidationKeysStore.

### Describe the solution you'd like

I would like the Microsoft.Extensions.DependencyInjection.IdentityServerBuilderConfigurationExtensions.AddApiAuthorization to not force the registration of the in memory ISigningCredentialStore and IValidationKeysStore if there is already one registered, or through the options.

### Additional context

Duende now suggests to use their automatic key management key store. It might be a good idea to leverage it by default instead of in memory implementation.