Add action support to build

[johnsca]

To support reactive actions, we should create boilerplate action executables based on the contents of actions.yaml during the charm build process.

[bcsaller]

Skeptical. Each file should generally be owned by one layer. This would be a generated file that should be hand edited. We could do this, but it is more of a template pattern than the current model we have.

[johnsca]

I don't think it should be hand edited at all. It should have the exact same contents as the hooks generated by interface layers. The custom code for the actions would live under reactive/ like all other reactive code.