feat(helm): Allow specifying podAnnotations per deployment

[mikutas]

Fixes #13389

Values added:

• destinationController.podAnnotations
  • annotations only for linkerd-destination
• identity.podAnnotations
  • annotations only for linkerd-identity
• proxyInjector.podAnnotations
  • annotations only for linkerd-proxy-injector

Each deployment's podAnnotations take precedence over global one by means of mergeOverwrite.

[adleong]

Thanks, @mikutas!

As we add more and more places in the Linkerd helm chart where users can insert their own customizations that really are unrelated to Linkerd's own function, I worry about the helm charts becoming bloated or inconsistent. This change itself looks good, but, for example, it adds a place to specify pod annotations but not pod labels. What about service labels and annotations? What about other resource types? As you can see, this would balloon out quickly if we wanted to be consistent and thorough.

I see 3 possible paths forward:

1. We encourage users who need to customize Linkerd's resources in a way that's not related to Linkerd's functioning to use something like kustomize so that we can keep Linkerd's helm charts clean and simple.
2. We add these values in a consistent way for all control plane resources and both labels and annotations (is there any other metadata that deserves this treatment too?)
3. We accept this PR (and PRs like it) to support users who want to make these customizations as they come up and acknowledge and accept that this will result in some non-uniformity and bloat in the Linkerd charts.

Let me discuss with the other maintainers and see what they think. In the meantime, please let me know if you have any thoughts about the above.

[adleong]

Thanks for your patience @mikutas. I've discussed with the other maintainers and we're happy to accept this PR.