enable User-defined templates to work with the gradle plugin #13705

Created by: Mintas

fix https://github.com/OpenAPITools/openapi-generator/issues/13705 (more detail on issue)

Client is expected to use files as Map<String, Map<String,String>>, where external key - is TemplateDefinition.templateFile, and internal map contains fieldName<>value mapping for other fields in TemplateDefinitions, thus keys are: folder (string, default ""), destinationFilename (string), templateType(string, name of TemplateFileType enum constant, default "SupportingFiles").

Example:

    openApiGenerate {
        generatorName = "spring"
        templateDir = file("templates").absolutePath
        files = [
            "my.mustache": ["templateType": "Model", "destinationFilename":"My.java"]
        ]
    }

for more info, see User-defined Templates Customization docs

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.