[DPE-3737] Add a zookeeper_client interface

[Batalex]

This PR adds a zookeeper interface.

Notes:

• The interface is zookeeper instead of zookeeper_client to keep compatibility with existing charms
• Tests are not implemented. I wrote a recap of stuff that don't work out in the following PR: [DPE-3737] Implement ZK client interface zookeeper-operator#142
  UPDATE: Turns out the tester design is not compatible with the way our charms update the relation databag (=not as a result of "zookeeper-relation-changed")

[marcoppenheimer]

Left some nitpicks and a couple suggestions! I haven't had a chance to go through with a fine-comb yet, but will do when the PR is open.

Overall it looks good! I haven't seen the Pydantic use for client-relation-interfaces yet, and I'm less familiar with what actually is expected for secret-relations, so I'll defer my opinions to those more in the know.

Oh, and I suggest you just open a branch in the repo, rather than using forks. Makes it easier whenever other people from the team want to make changes to a PR branch!

[Batalex]

Thanks for the review!

Oh, and I suggest you just open a branch in the repo, rather than using forks. Makes it easier whenever other people from the team want to make changes to a PR branch!

Unfortunately I don't have permission :(

[Batalex]

Hey there @gruyaume, I updated this PR with the new interface.yaml format. We have red CI due to other charms, should I re-build their json schemas?