Offer display permissions

[wallyworld]

Description of change

This PR builds on #7167
Please only review the second commit wallyworld/juju@6f26818
(Show offer permission in CLI output)

We now show the application offer permission a user has when running juju find-endpoints of show-endpoints.

QA steps

`$juju find-endpoints
URL Access Interfaces
admin/controller.mysql read mysql:db
admin/controller.foo consume mysql:db

$juju show-endpoints admin/controller.mysql
URL Access Description Endpoint Interface Role
admin/controller.mysql consume MySQL is a fast, stable and true multi-user, db mysql provider
multi-threaded SQL database server. SQL
(Structured Query Language) is the most
popular database query language in the world.
The ma...`

[axw]

LGTM, I think it would be best to have a unit test that covers the NotFound offer access case.

[wallyworld]

The mock was wrongly implemented - I fixed it to reflect the correct behavour and unit tests which exercise the functionality pass with the code change Also added a new bit of testing to the state package.

[wallyworld]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju