Add Gitlab provider

[michieljmmaas]

I added a Gitlab provider to find Gitlab Secrets.

The regex are sourced from this location: https://github.com/gitlabhq/gitlabhq/blob/master/gems/gitlab-secret_detection/lib/gitleaks.toml#L4-L51

Some things I was not sure about:

• The Doc of the provider file. Not sure what I should include there.
• I can extract common parts from the regex and create a method where you can input the variable parts (the prefix and the amount of chars). But I think doing it like this is easier to read.
• The test only tests for one of the regexes. I can add more, but the Github test also only tests for one regex.

Let me know what I can improve!

[sebastianfeldmann]

Nice, thanks