Build a configurable UserId service

[spion]

While writing the "Thinking in H4BFF" article, I realized there is no standardized way to get an identifier of the current user in the plugin. It might be nice if we provided this interface built in.

It would have to be configurable by the app though. Something like

this.getSingleton(UserProvider).setProvider(ctx => ctx.getService(MyUserAuthService).id);

Plugins will use this.getService(UserId).userId to fetch the current id, which will simply call the provider.

This will just provide a common unified interface so that you don't have to configure the user provider for each individual plugin. It will still allow you to build or use multiple user authentication implementations, so long as they set up the provider.

Concern: we might need to standardize the userId type to always be an uuid, or at least a string.