General improvements for Lanyard

[meppu]

⚠️ Since this pull-request replaces a lot of libraries, i wanted to contact phineas first to get his opinion. Since I didn't get any response i created pull-request anyway ⚠️

• Switched from Cowboy to Bandit. There are several reasons but these are the most important ones:
  • Bandit is written entirely in Elixir
  • Bandit's HTTP/1.x engine is up to 4x faster than Cowboy depending on the number of concurrent requests. When comparing HTTP/2 performance, Bandit is up to 1.5x faster than Cowboy.
• Switched from Poison to Jason. It is faster and new default for most of the libraries. And Poison didn't received any update in 2 years so it looks like it is currently unmaintained.
• Switched from HTTPoison to Finch. Finch is more minimal and performant.
• Added default avatar resolver for new usernames.

I tested these patches on my machine and tested functionalities like KV, REST API and WebSocket and seems like it all works great!

[Phineas]

Usually I'm not a huge fan of changing out libraries for the sake of it. Lanyard works very well and hasn't had downtime for over a year, so I'm hesitant to change something that works well.

However, I've done pretty extensive testing under this PR and haven't discovered any issues - plus it improves code readability, so I'll accept the PR. Thank you!