added wrapper functions for SignInWithEpic, SignInWithGoogle and SignInWithApple

[andygruening]

Docs Checklist

Please ensure you have addressed documentation updates if needed as part of this PR:

• I have created a separate PR on the sequence docs repository for documentation updates: Link to docs PR
• No documentation update is needed for this change.

[BellringerQuinn]

Thanks Andy! Code looks good to me, but unfortunately we can't test Epic sign in at the moment as the backend implementation is still being worked on

[andygruening]

Works as expected. It's hard to test it manually, but here's how you do it:

• Request access to the Epic Games dev portal from James or Taylan (required to login with the boilerplate)
• Clone the epic auth boilerplate and run the webpage and cloudflare worker locally: https://github.com/0xsequence-demos/epic-auth-login-boilerplate (change all variables to localhost and project keys. Not straightforward, feel free to ask me to get an id token)
• Change Unreal's project access key to one that has Epic Auth enabled
• Add a button to the WBP_Signin_Desktop widget and add an on click event
• On click, call SignInWithEpic from the SequenceSessionsBP subsystem
• You need to restart the unreal editor to see the signed in wallet session