feat: add tenantID template function

[FUSAKLA]

As discussed in this issue #3121 and as a followup from prometheus/alertmanager#3174 using the additional options.

I suppose we will wait for some AM release, not to import random commits?

Ad tests, I wanted to do some more on point test, but it's hard to make the AM to just render a template :/
So at least testing the option.

closes #3121

[FUSAKLA]

@gotjosh PTAL if you find a while since you have the context 🙏 (no rush just so it does not stay forgotten)

[pracucci]

Thanks for working on this! Overall LGTM. I left a couple of minor comments. Can you also add a CHANGELOG entry and rebase please?

The vendored alertmanager update adds Webex support. It should be also mentioned in the CHANGELOG.

[pracucci]

There are many vendor updates here. Are all of these required?

[FUSAKLA]

Unfortunately, it looks like it, or we would need to tweak it in the go.mod using some replace?
I suppose that was caused by the upgrade of the golang-lru in alertmanager :/

[pracucci]

@FUSAKLA I pushed a commit to add Webex support to Mimir. It was easier to do than explaining it 😉 I hope you don't mind!
41c55ad

[FUSAKLA]

@pracucci not at all, thanks for that!

Sorry about the delay, needed to take a break over holidays 🎄

Should be rebased now, added the changelog entry and resolved all the comments, thanks for looking into it.
There is still the question of the vendor updates, which I'm not sure how to deal with.

I also added a note about the templating to the docs in dceb21c

PTAL

[pracucci]

Thanks @FUSAKLA, LGTM! I just pushed a nit change to CHANGELOG and a smoke test in the config validation API. I hope you don't mind 🤗

[FUSAKLA]

Great, thanks!

[gotjosh]

Apologies for being late to the part but this looks very good to me! Thank you for your contribution @FUSAKLA.

[FUSAKLA]

No worries @gotjosh, thank you for guiding the AM work. That made it way easier :)