codeintel: Add auth support for GitLab

Created by: efritz

Currently on Cloud we only allow uploads for GitHub repositories. This PR adds gitlab auth checks to the LSIF upload endpoint. Fixes #31437 (closed). See https://github.com/sourcegraph/src-cli/pull/721 for the src-cli side of the change.

Test plan

Additional unit tests. Additionally, I validated upload works locally by setting "lsifEnforceAuth": true in site settings and uploading a github and gitlab index, without a token, with an invalid token, and with a valid token.

Works as expected.