[REQ] Allow generating only src/main/scala in sttp generator

The idea is very similar to https://github.com/OpenAPITools/openapi-generator/issues/6155. I would like to create a compiler plugin for sbt which generates only scala classes from openapi specification and puts them on the classpath.

As far as I got familiar with openApi generators while working on #6684 it seems to me that it might be quite easy to achieve that by just adding a single if statement into the sttpClientCode generator.

So the change would affect this method where we add templates to process:

    @Override
    public void processOpts() {
        super.processOpts();
        properties.forEach(p -> p.updateAdditionalProperties(additionalProperties));
        invokerPackage = INVOKER_PACKAGE.getValue(additionalProperties);
        apiPackage = API_PACKAGE.getValue(additionalProperties);
        modelPackage = MODEL_PACKAGE.getValue(additionalProperties);

        supportingFiles.add(new SupportingFile("README.mustache", "", "README.md"));
        supportingFiles.add(new SupportingFile("build.sbt.mustache", "", "build.sbt"));
        final String invokerFolder = (sourceFolder + File.separator + invokerPackage).replace(".", File.separator);
        supportingFiles.add(new SupportingFile("jsonSupport.mustache", invokerFolder, "JsonSupport.scala"));
        supportingFiles.add(new SupportingFile("project/build.properties.mustache", "project", "build.properties"));
        supportingFiles.add(new SupportingFile("dateSerializers.mustache", invokerFolder, "DateSerializers.scala"));
}

First of all I might be wrong and it is not so simple after all. The second question is whether it should be done for the rest of scala generators or even for all generators.

@clasnake (2017/07), @jimschubert (2017/09) heart, @shijinkui (2018/01), @ramzimaalej (2018/03), @chameleon82 (2020/03), @Bouillie (2020/04)

Start with the sttpClientCode generator's processOpts() method and the supporting files listed there; review issue #6155 and the related work in #6684 for context. Determine the intended scope across Scala generators, then verify that the requested mode generates only src/main/scala output suitable for an sbt classpath without the other listed supporting files.