[WIP] Add Julia client generator (beta)

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.3.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

For https://github.com/OpenAPITools/openapi-generator/issues/782

TODO:

• use camel case for file names
• replace "Swagger" with "OpenAPI" in the templates
• add style guide to contributing guideline: https://docs.julialang.org/en/v0.6.2/manual/style-guide/

credits @log0ymxm, @tanmaykm

To Julia pet store samples, please run ./bin/julia-petstore.sh or bin\windows\julia-petstore.bat