aws_completer: expose `help_text` for descriptions

**Is your feature request related to a problem? Please describe.**
Completions currently contain no descriptions.

**Describe the solution you'd like**
Additionally exposing the `help_text` from `CompletionResult` as `json` or `\t` separated (as in fish).
Enabled by another environment variable.

**Describe alternatives you've considered**
Accessing `create_autocomplete` [directly](https://github.com/rsteube/carapace-bin/blob/5ab20157b7041f0f22bee6b9ae122ac627baa504/completers/aws_completer/cmd/complete.py#L34) seems to work fine but that is incompatible with the binary release.

**Additional context**