[BUG] Inconsistent application of usePromise property in typescript-inversify templates

Created by: pcafstockf

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
[-] Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
Have you searched for related issues/PRs?
What's the actual output vs expected output?
[Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Inconsistent use of the usePromise additional property. When usePromise is specified, typescript-inversify templates generate Promises correctly in some places, but Observable in others.

openapi-generator version

5.3.1 Also exists in 5.4.x and 6.0.x branches.

OpenAPI declaration file content or url

Generation Details

java -jar openapi-generator-cli-5.3.1.jar generate -I input.yaml -g typescript-inversify -o outdir --additional-properties supportsES6=true --additional-properties usePromise=true

Steps to reproduce

Use any json or yaml file that declares a service/path with the above arguments. Inspect typescript-inversify/HttpClient.mustache and/or IHttpClient.mustache to see the problems.

Related issues/PRs

Suggest a fix

The fix is pretty simple. I will submit a PR for the 5.4.x branch shortly. Happy to submit to a different branch if need be.