feat(gitlab): support getDigest for gitlab repositories (#13361)

* feat(gitlab): support getDigest for gitlab repositories

* typehint and remove paging

* add gitlab to go.getDigest tests

* remove obsolete expectations

* add gitlab with a specific branch to go.getDigest tests

* exception handling and test for invalid/nonexisting commits

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>