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

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

NA

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

NA

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.

Reproduce with the provided openapi-generator command and a service/path specification, then inspect typescript-inversify/HttpClient.mustache and IHttpClient.mustache. Compare the generated Promise and Observable usage when usePromise=true; done means the templates apply that property consistently.