Allow to annotate pods on change using a different label

[rtnpro]

This change allows users to optionally specify a different label (than the one used to watch receiver pods) for annotating pods on hashring change.

This is useful in a separate Thanos receiver router and ingestor setup: https://thanos.io/tip/proposals-accepted/202012-receive-split.md/ where we need to watch a different set of ingestor pods to update the hashring, but the hashring is consumed by a different set of router pods.

Fixes #125

[philipgough]

@rtnpro would you mind rebasing on main

[rtnpro]

@philipgough I have rebased this PR on top of latest main