V4: State and Cache providers

[quinchs]

Summary

This PR implements state and cache providers as proposed in #2238.

The current api of the providers is up for debate, I've implemented it how I see fit but feedback and reviews are always welcome.

Todo:

• [x] API design
• [x] Internal reference cache
• [ ] Default providers
  • [x] Default concurrent providers
  • [ ] Redis cache provider
• [ ] Entities
  • [x] Users
  • [x] Presence
  • [ ] Channels
  • [ ] Guilds
  • [ ] Messages
  • [ ] Emotes

[AddressXception]

Is this still on the roadmap? I'm building something and using this library but running into a few limitations that I need to work around. I left my comments in this discussion here:

https://github.com/discord-net/Discord.Net/discussions/2508

thanks!