Support for new relation-model-get hook tool

[benhoyt]

Per our discussions with the Juju and Observability teams, Juju 3.6.2 will be coming out with a new hook tool, relation-model-get (Juju code change). It returns the model info (just the UUID right now) of the model on other side of the relation. You can -- among other things -- compare it to the current model UUID to determine whether the relation is cross-model or not. The hook tool works as follows:

# -r required if not in a relation hook context
relation-model-get [-r <relation-id>] [--format yaml|json]

# example:
$ relation-model-get -r 3 --format json
{"uuid":"deadbeef-0bad-400d-8000-4b1d0d06f00d"}

We'd like to add first-class support for this in Ops. Presumably this will belong on the ops.Relation object. Perhaps a remote_model property which returns a RemoteModelInfo dataclass with a uuid: str field. (I thought about returning a Model instance, but that has a ton of other stuff, so seems wrong to type it as that.) That would call relation-model-get -r {self.id} --format json and cache it on the Relation instance.

It's an open question what we should do if the Juju version is too old to support relation-model-get. I'd recommend just have it raise the ModelError. Alternatively we could have remote_model typed as RemoteModelInfo | None, but that doesn't seem right in the long term. We could also consider adding a property to JujuVersion like supports_relation_model_get -- worth talking to charmers about this.

[benhoyt]

@wallyworld Ops tracking item for your info.

[benhoyt]

Per Ian, if a charm needs this feature it'll just say assumes: juju>=3.6.2 (or whatever the syntax) in charmcraft.yaml, so we don't need special handling in Ops.