[GH-678] Fix bot authentication

[AminArria]

Motivation

Closes #678

Summary of changes

• Create endpoint /auth/generate-bot-token in gateway to generate tokens for bots to use
• Create a gen_server TokenFetcher in bot_manager to generate a JWT every 30 minutes (token expires ~2 hours)
• Remove hack in arena GameSocketHandler to force any new connection to have either the user query param (gateway_jwt) or the bot query params (bot_token and bot_secret)

How to test it?

Play the game and if bots keep working then this is working 😄

Checklist

• Tested the changes locally.
• Reviewed the changes on GitHub, line by line.
• This change requires new documentation.
  • Documentation has been added/updated.