feat: allow oauth2 authentication

[bastiandoetsch]

Description

This PR overhauls the authentication in Language Server.

1. By default, it registers OAuth2 as primary provider and Token (CLI) Authentication as secondary. If OAuth2 fails, it automatically falls back to token authentication.
2. By specifying token as authentication method during initialization, or as configuration update, Token-only authentication can be forced.
3. The whole authentication & initialization code has been moved to streamline dependencies (top->down). Authentication does not have backlinks to the snyk domain anymore.

Checklist

• Tests added and all succeed
• Linted
• README.md updated, if user-facing
• License file updated, if new 3rd-party dependency is introduced

🚨After having merged, please update the CLI go.mod to pull in latest language server.

Screenshots / GIFs

Visuals that may help the reviewer. Please add screenshots for any UI change. GIFs are most welcome!