feat: add group ID option

[nico151999]

The intention is to make group IDs configurable. I opened an issue in order to target this. Before working on the respective changes in the linkerd2 repo I thought it would make sense to get this feature implemented here first.

This pull request adds the option to set the group ID via command line and via annotation. The tests available via just ran successfully in the dev container and some manual tests also ran well. I had to tweak my setup a little bit in order to make it work in WSL2 behind an HTTP proxy, so my setup might differ from the reference setup. Still, I assume that my tweaks did not have an impact on the outcome of the tests. It is also worth mentioning that tweaking IP tables manually is not part of my daily business. I just mention it for the reviewers to pay special attention. Thanks in advance for the review. Looking forward to this feature getting merged.

[mateiidavid]

@nico151999, I'm sorry it took me so long to review this. This looks good! I tested it out in the control plane and there are no side effects (that I've noticed).

I'll ack & merge it if you can do a rebase. We've changed some proxy-init code (including making the package public, so I anticipate it won't be a super clean merge). If CI's green and happy, we're good to go.

[nico151999]

@mateiidavid I probably won't get round to it for the next few days, but yes, I will do a rebase, probably some time next week. Nice to see some feedback. Once this is merged I would like to ask you to have a look at my PR in the main linkerd2 repo, too (as linked in the issue). It will probably need a rebase, too, but I would like to have the attention of someone who can approve the changes so that I won't refactor it redundantly and need to do yet another rebase then. Anyway, let's first get the changes to proxy-init in. Thanks!

[nico151999]

@mateiidavid rebased 😄

[mateiidavid]

Thanks @nico151999!