Allow apps that don't want to have a dedicated activity to set a DigitalCredentialsPresentmentMechanism from intent

[asghol]

Currently you need to extend a specialized activity class (CredentialManagerPresentmentActivity) to be able to make use of the code that handles the incoming credential request when a GET_CREDENTIAL intent action comes in to the app. To be usable for apps aiming for single activity styled apps or apps with custom visuals, we could extract that part and expose it so that those types of apps don't need to manually extract that part.

[davidz25]

With #1510 we changed how presentment works and if you look at the current CredentialManagerPresentmentActivity.kt file it should be a lot easier to follow what's going on since it's all linear and basically just one call to digitalCredentialsPresentment() ...

With that in mind, would it make sense to factor some of the functions used in CredentialManagerPresentmentActivity as utility functions so you can use these in your own activities? Let me know here in the bug what you'd like, happy to facilitate this.