Model introspection for units

[aieri]

I see that the ops.Model class allows units to introspect the model they're in, but only in limited ways.
Allowing units to have a (read-only) full view of the model would allow charms to make more informed decisions about their behaviors. For example, applications that should be ubiquitous like grafana-agent would be able to tell if they have been deployed on some but not all machines and warn the user accordingly, without the need for external tooling.

In other words, the ops module could provide an idiomatic way for charms to consume juju export-bundle without awkward interactions with the juju api.