OpenAPITools/openapi-generator support

[AlexTrotsenko]

Is your feature request related to a problem? Please describe.
Many apps uses Swagger/OpenAPI Specification and OpenAPITools codegen when communicating over http to BackEnd.

Currently migrating existing Retrofit-based app to Ktorfit seems relatively easy task.

However it can't be done for the apps, which are using auto-generated retrofit interface from Swagger definition: manual change is not possible since normally Swagger definition is being regularly updated.

Describe the solution you'd like
Add Ktorfit library support (should be similar to the existing jvm-retrofit2) to the Kotlin generator.

Describe alternatives you've considered
There is old multiplatform library option, but that one uses "raw" Ktor api and seems to be not very up-to-date (e.g. Ktor 1.6.7 client is mentioned in the docs).

Additional context
As far as I see, simple mustache templates are used to customise code generation. Existing jvm-retrofit2 and multiplatform options could be used as the starting point.

E.g. Example of adding kotlinx-datetime to the "old" multiplatform codegen
https://github.com/OpenAPITools/openapi-generator/pull/17166/files.