provide flag value autocomplete for --label

[heyjcollins]

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 --label.

When I've typed --label , then press <TAB>, all the current label keys already associated with my workload AND all the possible label keys associated with each supply chain (which haven't already been added to the workload) on the target cluster are auto-suggested.