Add a version gate endpoint

[F1F7Y]

Add a version gate endpoint so that third-party applications can know what it's set to.

Possibly an extension of #10

[GeckoEidechse]

Could also expose it in https://northstar.tf/metrics in the meantime ^^

[pg9182]

That's doable @F1F7Y. I'll add one when I'm back next week.