gitaly: Add config option to enable transactions

What does this merge request do and why?

gitaly: Add config option to enable transactions

Support for running Gitaly with transactions/WAL enabled was added with gitaly!6496 (merged). This is currently experimental and intended for testing/development use only.

Add a configuration option to gdk.yml and associated documentation to allow users to enable Gitaly transactions in GDK.

How to set up and validate locally

1. Enable transactions in gdk.yml:

gitaly:
  enable_all_feature_flags: true  # enables Git v2.44, required for transactions
  transactions:
    enabled: true
praefect:
  enabled: false

2. gdk reconfigure && gdk restart
3. Run gdk tail gitaly and ensure the following log appears:

2024-05-01_23:26:53.49498 gitaly                : {"level":"warning","msg":"Transactions enabled. Transactions are an experimental feature. The feature is not production ready yet and might lead to various issues including data loss.","pid":13727,"time":"2024-05-01T23:26:53.494Z"}

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.