Authentication: Authorize using stateless JWT tokens

🧩 Problems to solve

We need to allow authentication and authorisation using stateless JWT tokens as part of Web IDE integration. The existing OAuth and PAT authentication methods should still work for VS Code Extension and other integrations.

💡 Proposal

This will be about creating a new authentication method that accepts the JWT token from the Web IDE, and authenticate the request to code suggestions.

Block highlighted in #e9967a is part of this issue.

👣 Implementation order

• GitLab API endpoint to issue token - gitlab-org/gitlab#404427 (closed)
• (This) Model Gateway authentication with JWT token - #56 (closed)
• Authenticate Web IDE to Model Gateway - gitlab-org/gitlab-web-ide#140 (closed)

📔 References

1. Solution validation issue - gitlab-org/gitlab-web-ide#141 (closed)