Add an Action that invokes a different completer

[rsteube]

for situations like gh repo clone -- <git clone completion>

• short-term solution: Action that invokes a completer from within carapace-bin (or carapace in general using json export?)
• long-term solution: Action that invokes what the shell provides with https://github.com/rsteube/invoke-completion

related #898

[rsteube]

specs are now invoked with bridge.ActionCarapaceBin, other completers can thus be embeded with macros