[UNREAL] Add Steam and Epic Games Account Federation to Beamball Sample

[PedroRauizBeamable]

Today, we have a Steam sample --- that is going away.

Instead, we'll have the various Console/PC identity federation samples all centralized in the Beamball Sample.

This task is basically:

• Move the Steam Federation Sample to the Beamball sample and add a dedicated page about it to the docs.
• Add Epic Games Federation to the Beamball sample and add a dedicated page about it to the docs.
• Modify the Main Menu UI such that:
  • Players can sign-in/up with email/password.
  • Players can choose to sign in with Steam/Epic.