storage

[PietroPasotti]

This PR implements storage mocks for Scenario and adds API to State and Context to work with them.

from scenario import Storage, Context, State

# some charm with a 'foo' filesystem-type storage defined in metadata.yaml 
ctx = Context(MyCharm)
storage = Storage("foo")
# setup storage with some content
(storage.get_filesystem(ctx) / "myfile.txt").write_text("helloworld")

with ctx.manager("update-status", State(storage=[storage])) as mgr:
    foo = mgr.charm.model.storages["foo"][0]
    loc = foo.location
    path = loc / "myfile.txt"
    assert path.exists()
    assert path.read_text() == "helloworld"

    myfile = loc / "path.py"
    myfile.write_text("helloworlds")

# post-mortem: inspect fs contents.
assert (
    storage.get_filesystem(ctx) / "path.py"
).read_text() == "helloworlds"

Feature creeps:

• _EntityStatus now supports subclass checks: isinstance(State.unit_status, ops.ActiveStatus)
• State.planned_units mock

[benhoyt]

This looks good to me! API seems consistent with the rest of Scenario.

Just left a couple of style nit comments.