Support GitLab Tokens for Bot Access

[melotte25]

Instead of requiring a single user's OAuth token to interact with your repositories, supporting GitLab's Group, Project, and/or Personal Access Tokens would be beneficial.

Using an account to generate an OAuth token with Codecov for the bot means:

1. A license must be used at GitLab purely for authenticating to Codecov (if you want a "bot" account)
2. If you want to save a license, you can use a user account, but then messages from Codecov are posted as that user which leads to confusion and ambiguity when reading MRs.
3. The OAuth expires without notice, so when using a GitLab user account, one day Codecov will just break without notification until after multiple days of having Codecov Support and my team troubleshoot the issue only to realize that just re-authenticating with the GitLab "bot" account to Codecov gets a new OAuth fixes everything, which leads to wasted resources and poor CX.
4. Goes against "general" security practices of token management and expiration for application integrations (managing a token fits in well to existing processes for 3rd party integration)

Thanks for listening!