Expose `pause` and `resume` methods of `kamal-proxy`

[baldarn]

I created the issue to be addressed https://github.com/basecamp/kamal/issues/1046

The idea is to expose the pause and resume commands of the kamal-proxy

Proxy Interface edits

This pr enables that via the some new commands in the proxy interface:

• pause_app will triggers the pause command in all kamal proxy containers
• resume_app will triggers the resume command in all kamal proxy containers

Concerns

I feel like the presence in the proxy interface is not the best option, what do you think? i feel like this could go in the app interface, not really sure

Tests

tests of all levels are there. i found some old ugly tests in the integration part, but managed to have something running ;)

this could be useful to address a maintenance mode as requested here https://github.com/basecamp/kamal/issues/162

Docs

docs pr https://github.com/basecamp/kamal-site/pull/122