Added converterFactories property to ApiClient in jvm-retrofit2.

[dylan-kwon]

The existing converterFactory property worked fine, but was not available in certain cases.

I think there are these problems.

1. You can add only one converterFactory.
2. converterFactory can only be added at the very end.

In my case, I had to add my custom Converter.Factory first, because JsonParser doesn't handle empty strings and throws an exception. (#14982)

At first, I planned to add a preConverterFactory property, but changed converterFactory to an array to make it more customizable.

For backwards compatibility, converterFactory has not been removed from the code, it has been left using the @deprecated annotation.

I think this PR of mine will help you meet more requirements.

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@jimschubert, @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @wing328

[dylan-kwon]

Hi, @4brunu

ci/circleci: node3 — Your tests failed on CircleCI Any idea what could be causing this to fail?

[4brunu]

Looks good to me

[4brunu]

CI failure it's not related to this PR.

[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.2.1:exec (npm-test) on project TypeScriptBrowserPetstoreClientTests: Command execution failed.: Process exited with an error: 1 (Exit value: 1) -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
[ERROR] 
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <args> -rf :TypeScriptBrowserPetstoreClientTests