Helper for operator-framework focused interface libraries to be used in reactive charms

[johnsca]

Library to allow interface API libraries designed primarily for the operator framework to be registered with the legacy reactive framework so that they can be used in reactive charms with minimal effort on the interface API library maintainer (primarily just by implementing a manage_flags method which will only be called when used in a reactive charm). The docs now contain a complete, albeit simple, example of how this is intended to be used.

This depends on canonical/charms.reactive#227 (but can use it from the branch until it's released).

Things that are missing / known to be broken:

• README / documentation
• Entry points don't work in reactive K8s operator charms #2 This does not currently work with reactive K8s operator charms due to issues with layer:caas-base
• Due to Entry points don't work in reactive K8s operator charms #2, we should also include integration tests using K8s charms (they also tend to be faster and more reliable, if trickier to debug)

[johnsca]

I'll have to sort out the GitHub Workflow issue for the integration test, but I did get a passing run locally, at least:

========== 1 passed in 753.61s (0:12:33) ==========
_____________________ summary _____________________
  integration: commands succeeded
  congratulations :)