[dev] Include exposed endpoints in diff bundle output

[achilleasa]

Description of change

This PR ensures that the juju diff-bundle command reports exposed endpoint differences in its output.

The diff-bundle logic fetches an overview of the model from the controller via the FullStatus method of the Client facade. Its response payload (ApplicationStatus) has been augmented with an extra, optional field to report the set of expose settings (if any) for the application endpoints.

QA steps

$ juju bootstrap lxd test --no-gui

$ juju deploy apache2
$ juju expose apache2 --endpoints website --to-spaces alpha --to-cidrs 10.0.0.0/24
$ juju show-application apache2
apache2:
  charm: apache2
  series: bionic
  channel: stable
  principal: true
  exposed: true
  exposed-endpoints: <----- you should see this block as part of the output
    website:
      expose-to-spaces:
      - alpha
      expose-to-cidrs:
      - 10.0.0.0/24
  remote: false
  endpoint-bindings:
    "": alpha
    apache-website: alpha
    balancer: alpha
    local-monitors: alpha
    logging: alpha
    logs: alpha
    nrpe-external-master: alpha
    reverseproxy: alpha
    vhost-config: alpha
    website: alpha
    website-cache: alpha

# Check 1: 2.8-compatible bundle without expose flag
$ cat bundle1.yaml
applications:
  apache2:
    charm: cs:apache2-35
    series: bionic
    num_units: 1
    to:
    - "0"
machines:
  "0":
    series: bionic

$ juju diff-bundle bundle1.yaml
applications:
  apache2:
    expose:
      bundle: false
      model: true
    exposed_endpoints:
      website:
        bundle: null
        model:
          expose_to_spaces:
          - alpha
          expose_to_cidrs:
          - 10.0.0.0/24

# Check 2: 2.8-compatible bundle WITH expose flag (implicit expose-all-to-0.0.0.0/0)
$ cat bundle2.yaml
applications:
  apache2:
    charm: cs:apache2-35
    series: bionic
    expose: true
    num_units: 1
    to:
    - "0"
machines:
  "0":
    series: bionic

$ juju diff-bundle bundle2.yaml
applications:
  apache2:
    exposed_endpoints:
      "":
        bundle:
          expose_to_cidrs:
          - 0.0.0.0/0
          - ::/0
        model: null
      website:
        bundle: null
        model:
          expose_to_spaces:
          - alpha
          expose_to_cidrs:
          - 10.0.0.0/24

# Check 3: 2.9-compatible bundle with endpoint-specific expose settings
$ cat bundle3.yaml
applications:
  apache2:
    charm: cs:apache2-35
    series: bionic
    num_units: 1
    to:
    - "0"
machines:
  "0":
    series: bionic
--- # overlay
applications:
  apache2:
    exposed-endpoints:
      "":
        expose-to-cidrs:
        - 0.0.0.0/0
      website:
        expose-to-cidrs:
        - 192.168.0.0/24

$  juju diff-bundle bundle3.yaml
applications:
  apache2:
    exposed_endpoints:
      "":
        bundle:
          expose_to_cidrs:
          - 0.0.0.0/0
        model: null
      website:
        bundle:
          expose_to_cidrs:
          - 192.168.0.0/24
        model:
          expose_to_spaces:
          - alpha
          expose_to_cidrs:
          - 10.0.0.0/24

[achilleasa]

This pattern where the base is the latest version feels really odd!

[achilleasa]

@SimonRichardson I have rebased this PR to include the IPV6 changes that landed on develop and updated the QA steps accordingly.

[SimonRichardson]

I'm not sure of the rules around Client facades, the code looks fine though and Q&A went well, so until I get clarification about it, I'm just going to leave a comment for now.

[achilleasa]

The PR has been updated to revert the versioning changes and instead just add an extra field (with omitempty) to the existing payload.

[achilleasa]

$$merge$$