feat: Add friends and game invites

[touficbatache]

Linked to touficbatache/SecretHitlerSvelte#3

How it could potentially work:

1. No followers/following complexity => just friends or not: send friend request, if accept then both are friends.
2. Use Firestore to store friendships in the form of documents:
   a. Document name: "FRIEND1_FRIEND2" where FRIEND1 would be asker ID and FRIEND2 receiver ID.
   b. Document contents: status of request, sent date, accepted date, common game IDs (maybe not needed cuz we can query it from Realtime DB anytime) to have a list of common games and count (maybe even make friendship badges?)
3. Call API to send invite => use Realtime DB and listen (need to store them temporarily and need rapid access)
4. Call API to accept invite => update Realtime DB (remove invite) and join game
5. Add ability to cancel invite by calling API

Originally posted in touficbatache/SecretHitlerSvelte#3 (comment)