[REQ] Allow user to add custom mustache template for adding/generating user specific code

Problem

There is no provision to add custom logic or code other than modifying the existing mustache files. Users might want to add additional logic by creating their own mustache file, which might need a provision from the generator to support it.

This could be an optional feature but will generally useful to make custom template code generation to suit the needs of users generating the client whose has their own specific requirements.

proposed solution

The possible solution would be to give user provision to add his own custom template (.mustache) file and which will be processed while generating the client. may be from the generation command as the command line args or additional-properties key

Ex:
java -jar openapi-generator-cli.jar generate --input-spec https://..../../reference.json --generator-name r --output OutFolder --template-dir Utilities/templates --additional-properties customTemplate=custom.mustache

In the above command i have passed in custom.mustache as additional property with the key name as customTemplate, which might generate a custom.R (for R langauge client) file.

need suggestions

like to have discussion if there is a better way or approach of adding/implementing this keeping other language generation process in consideration.
@wing328

Start by tracing the generate command's --template-dir and --additional-properties inputs, then compare how templates are processed across generators. No files or tests are named; done would require an agreed design and support for an optional custom .mustache file that can produce a custom R client file without disrupting other language generators.