feat: migrate model user permissions

[Aflynn50]

Import and export model user permissions. In mongo there was the concept of model users, these have been reduced to model user permissions in 4.0 since the controller holds all extra information about the users.

Checklist

• [x] Code style: imports ordered, good names, simple structure, etc
• [x] Comments saying why design decisions were made
• [x] Go unit tests, with comments saying what you're testing
• [x] Integration tests, with comments saying what you're testing
• [ ] doc.go added or updated in changed packages

QA steps

Test 4.0 to 4.0

juju bootstrap lxd source
juju bootstrap lxd target
juju switch source
juju add-model test
juju add-user jim
juju add-user bob

# Grant some permissions on the model
juju grant jim read test
juju grant bob write test

# Generate some last login times
juju change-user-password admin
juju change-user-password jim
juju change-user-password bob
juju logout && juju login -u jim
juju status
juju logout && juju login -u bob
juju status
juju logout && juju login -u admin

# Start migration
juju show-model test

juju switch source
juju 

Test 3.6 to 4.0

# Install 3.6
$ juju bootstrap lxd test36
$ juju add-model test-model 

# Repete steps above to add users and generate last login times

$ juju show-model test-model-users
test-model-users:
  name: admin/test-model-users
  short-name: test-model-users
  model-uuid: 02138c75-326d-4f0c-8d30-e45ae6e5f6c1
  model-type: iaas
  controller-uuid: 42a80563-f6cf-4d02-8e53-73e888bb3711
  controller-name: test36
  is-controller: false
  owner: admin
  cloud: lxd
  region: default
  type: lxd
  life: alive
  status:
    current: available
    since: 3 minutes ago
  users:
    admin:
      display-name: admin
      access: admin
      last-connection: just now
    bob:
      access: write
      last-connection: 1 minute ago
    jim:
      access: read
      last-connection: 28 seconds ago
  sla: unsupported
  agent-version: 3.6-beta3.1
  credential:
    name: lxd
    owner: admin
    cloud: lxd
    validity-check: valid
  supported-features:
  - name: juju
    description: the version of Juju used by the model
    version: 3.6-beta3.1

# Switch to target 4.0 controller
$ juju switch target
$ juju migrate test36:test-model-users target
# Wait for migration to complete
$ juju show-model test-model-users
test-model-users:
  name: admin/test-model-users
  short-name: test-model-users
  model-uuid: 02138c75-326d-4f0c-8d30-e45ae6e5f6c1
  model-type: iaas
  controller-uuid: 294ed49a-d474-4e0a-8de2-15052e921a93
  controller-name: target
  is-controller: false
  owner: admin
  cloud: lxd
  region: default
  type: lxd
  life: alive
  status:
    current: available
    since: 34 seconds ago
  users:
    admin:
      display-name: admin
      access: admin
      last-connection: 2 minutes ago
    bob:
      access: write
      last-connection: 3 minutes ago
    jim:
      access: read
      last-connection: 2 minutes ago
# Do an upgrade model to check its all fine
$ juju upgrade-model
juju show-model admin/test-model-users
test-model-users:
 ...
  users:
    admin:
      display-name: admin
      access: admin
      last-connection: 3 minutes ago
    bob:
      access: write
      last-connection: 12 minutes ago
    jim:
      access: read
      last-connection: 12 minutes ago
...
  agent-version: 4.0-beta5.1
...
  supported-features:
  - name: juju
    description: the version of Juju used by the model
    version: 4.0-beta5.1

# Login with bob to check he can see the controller

$ juju logout
$ juju login -u bob
$ juju show-model admin/test-model-users
...
  users:
    bob:
      access: write
      last-connection: 16 minutes ago
...

Links

Jira card: JUJU-6512