Allow gitaly configuration to use transactions

What does this MR do and why?

With gitaly!6496 (merged) gitaly will be able to use transaction management. As this is will be the preferred mechanism for running gitaly let's enable the ability to configure this for testing gitaly#5664 (closed)

The changes in this MR will allow for setting the configuration based on an environment variable, which we can subsequently set in the pipeline as needed.

The follow up MR to enable this in the pipelines is: Support gitaly transactions pipeline (gitlab!152388 - merged)

Screenshots or screen recordings

• Sample Pipeline: pointing at Support gitaly transactions pipeline (gitlab!152388 - merged)
  • praefect 1/2
  • praefect 2/2
  • gitaly-transactions 1/2
  • gitaly-transactions 2/2

How to set up and validate locally

• Check the pipeline jobs logs for the gitaly service and ensure a log line exists Transactions enabled. Transactions are an experimental feature. The feature is not production ready yet and might lead to various issues including data loss. See https://gitlab.com/gitlab-org/gitaly/-/blob/348c0477123886ba100b17e1858340d5453adca0/internal/cli/gitaly/serve.go#L369

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.