Allow annotations on actions

[Marco Ceppi]

A feature we're developing for benchmarking (which is implemented as actions) is to parse information about the benchmark. Currently we store this in a database as part of a webservice deployed as a charm (like Juju GUI). However, as we evolve the command line plugins for benchmark we have no way to decouple requiring the GUI to use the CLI. If we could set this data (very simple key values) as an annotation on the action it would mean we would only need to parse data once by GUI or CLI and it'd be attached to the action for further lookup. Much in the way that gui-x and gui-y are annotations for services in the Juju GUI