provide flag value autocomplete for `--env`

[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 --env

When I've typed --env then press <TAB>, all the current env vars keys associated with my workload are auto-suggested