feat: interpret auth tokens

[boltlessengineer]

Implement Basic and Digest authorization tokens which are both supported in intellij's http client.

Fixes #483

• Implement Basic authorization
• Implement Digest authorization
  • Refactor to make chained requests possible
  • Refactor to correctly parse chained response (trace)
  • Actually implement the Digest authorization.

Edit: add todos to show progress

[boltlessengineer]

Basic is easy, but implementing Digest seems to require some work as rest.nvim needs to send multiple requests in same go.

[stephenprater]

I think Basic is way more common that Digest - any chance we could go ahead and merge the Basic code path and come back to do Digest?