Add mTLS scenario

What does it do

I have implemented a scenario that creates two docker container, which are hosting a gitlab instance (with gitaly disabled) and another gitlab instance (with everything but gitaly disabled). The main feature here is that both instances use TLS and thus create a mTLS connection.

Main changes

To be able to have mTLS, I had to add an additional certificate as well as create a trusted-certs folder. All of this is added to the instances with volumes.

Test

Since there is no test that currently uses this, I created a test that uses the new tag and creates commits (content pushed to gitaly) with different approaches. To be able to see the results of the test in combination with this MR, I had to start a new pipeline to see the effects.