Praefect support for TLS

Problem to solve

Gitaly supports TLS to secure data in transit. When Praefect is put in between the two components this capability is lost and data is insecurely transmitted again.

For customers who want TLS to secure communicate between GitLab components this is a requirement.

Proposal

This issue is aimed at making sure each client can talk securely to praefect, and it itself connects securely to Gitaly. Further, it should be checked if the relevant documentation is up to date and accurate.