Add interface schemata and docs for service-mesh interfaces

When we landed the service-mesh interface libraries, we didn't add interface docs and metadata at that time. #533 highlights the need for accurate interface metadata -- in particular, we need to be sure we have the canonical interface name exactly right.

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The libraries touched by #533 should all have the following files added at minimum:
- `interface/v0/README.md`
- `interface/v0/interface.yaml`

These should follow the format of the files found in https://github.com/canonical/charmlibs/tree/main/interfaces/istio_metadata/interface/v0

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Ideally we'd also add Pydantic schemas at the same time. These should follow the lightweight format of https://github.com/canonical/charmlibs/blob/main/interfaces/istio_metadata/src/charmlibs/interfaces/istio_metadata/_schema.py

They don't need to be symlinks into the libraries (though they could be) -- copies of any schema classes from the libraries are perfectly acceptable.

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While these files can all technically be added with only review from the service-mesh team as codeowners of the libraries, I'd appreciate being pinged for review on them too.