[Apex] migrating to OpenAPI Generator project

[wing328]

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, 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

• cherry-pick the commit by @muenzpraeger in Apex #687
• commented out unused code
• left comments about revising code
• Ran ./bin/apex-petstore.sh without issues.

[wing328]

TODO

• rename "Swag" model prefix to "OAS" or something else
• rename file names (e.g. SwaggerTest.cls => OpenAPITest.cls)
• review the code block (commented out) in https://github.com/OpenAPITools/openapi-generator/pull/692/files#diff-81ebe961e09794dde5304932869f20bdR568
• test the output

[wing328]

As discussed with @muenzpraeger we'll merge this first and then he will submit PRs to further enhance the Apex client generator