[BUG] Typescript/typescript-angular missing import for interfaces generated from additionalProperties

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

Hi there, I believe I found an issue with generated typescript code for the typescript-angular generator (but I'm also hoping it is configuration.).
I have posted a full example with generated code, it calls compodoc (which attempts to parse all services)
Typescript missing imports for interfaces generated from additionalProperties

openapi-generator version

4.1.2
"@openapitools/openapi-generator-cli": "0.0.19-4.1.2"

OpenAPI declaration file content or url

https://github.com/ltomes/openapi-generator-issue/blob/master/specWithAdditionalProperties.yaml

Command line used for generation

./node_modules/.bin/openapi-generator generate -i ./specWithOutAdditionalProperties.yaml -g typescript-angular -o src --package-name restClient --enable-post-process-file --additional-properties=npmName=restClient,supportsES6=true,withInterfaces=true,enablePostProcessFile=true

Steps to reproduce

I have created a minimal repo to reproduce the issue: https://github.com/ltomes/openapi-generator-issue

1. npm install, any node 10+ version
2. npm run build:additional Observe that the generated code (src/api/default.service.ts) is missing any imports to additionalProperties from the schema/they are not imported as type interfaces.

Related issues/PRs

I see other users getting generation issues now and again, but I don't see this exact issue reported.

Suggest a fix

additionalProperties should be evaluated for imports the same way properties are (Or I have misconfigured the cli, which I would love to find out/address)

It seems that our imports object here is missing imports defined from additionalProperties?

Start with modules/openapi-generator/src/main/resources/typescript-angular/api.service.mustache, especially the imports object near line 33, and compare it with the generated src/api/default.service.ts in the linked reproduction. Run npm install followed by npm run build:additional; done means interfaces produced from additionalProperties are imported and the generated TypeScript builds successfully.