add support to Maven proxy repository for gitlab private maven repository http header token authentication

Gitlab repository only provides authentification using custom HTTP Headers, for example Private-Token

https://docs.gitlab.com/ee/user/packages/maven_repository/#authenticate-to-the-package-registry-with-maven
 

Adding "Custom fragment to append to "User-Agent" header in HTTP requests." not works

Gitlab documentation states:

If a project is private or you want to upload Maven artifacts to GitLab, credentials will need to be provided for authorization too. Support is available for personal access tokens, CI job tokens, and deploy tokens only. Regular username/password credentials do not work.

Expected

We mainly want to have special HTTP header authentication working with gitlab remotes.

Though a possible impl. would be to allow a new proxy repository option to specify adding custom outbound HTTP headers and values for every method of request - though that might be overkill on further analysis, depending how/if gitlab sends auth challenges.