pass commit sha of the last update

[passichenko]

Context

For observability purposes we need to have a way to reference a specific version of the config. For example for tracking rollouts or to see what value was used during debugging.
We already have sha field which is git-style sha of the content, but it's hard to lookup the config content using it (unless we build some additional tooling).
We also report commit_sha in many places, but it's global for the repo, so it will change if there are new commits even if those commits didn't modify the specific config that we are looking at.

Change

I propose to use sha of the last commit that modified the config as it's version. We already know it on rollout, we just need to store it and pass it around. The only issue is with local git mode as we'd need to use git log to get sha of the last update for every config. I think it should be fine, but worst case we can just not report it with local git setup.

[passichenko]

I'm torn between naming it explicitly like that and naming it more generic like version so it's easier to change format later.

[DavidSGK]

I think keeping the explicit name is better personally - for forward compatibility, we can follow protobuf best practices for marking deprecated, etc.