Added Julia client and server generators

[tanmaykm]

This PR adds two new generators for the Julia language

• julia-client to generate a client from specifications
• julia-server to generate a server with stubs that can be used to host a server conforming to the specifications

The generated code uses the Julia OpenAPI.jl package that includes support functions for both client and server.

Also fixes #782.
Similar to #1084, but uses OpenAPI.jl which supercedes the earlier Swagger.jl implementation.

cc: @wing328

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.

[tanmaykm]

Thanks @wing328 for taking a look.
I have rebased with master and added b4042b4 to fix errors in the last ci run.

[tanmaykm]

Looks like I need to rebase and run bin/utils/ensure-up-to-date as advised in the failure report?

[tanmaykm]

Done that now in 1889ce0

[wing328]

@tanmaykm thanks for the PR to add the new generator. I'll file another PR to make some minor improvements.

Can you please also PM me via Slack when you've time?

https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g

[tanmaykm]

Thanks! Will do.