provide flag value autocomplete for --service-ref

[heyjcollins]

Description of problem

app devs appreciate efficiency and convenience.
providing broad support for autocompleting flag values makes it possible for people to NOT remember names of items added previously by them or others.

providing autocomplete for flag values inconsistently is a bad UX and we should do all that we can to consistently deliver on the contract.

Proposed solution

apps plugin already supports autocomplete for some flag values (e.g. --namespace --- will return list of possible namespaces)

Add similar support for --service-ref

When I've typed --service-ref then press <TAB>, all the current service-ref keys associated with my workload should be displayed AND (if possible) a list of any services available to be bound to the workload (excluding any service(s) already bound to the workload).

If I select an available service, I should be prompted to provide a key-name for the service, and once provided, the plugin should add a properly formatted key/value --service-ref flag/value for me.

[heyjcollins]

@danfein - added "needs-design" here because I'd like your input on the somewhat more complicated autocompletion interactions described here.