Add new Go client (experimental) generator

[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, ./bin/openapi3/{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\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

Add a new Go client (experimental) generator

This generator may break and subject to breaking changes without further notice.

The goal is to allow enhancements (breaking changes) to be tested more rapidly before 5.0.0 release (including breaking changes without fallback) in May 2020.

cc @antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09

[wing328]

cc @bkabrda as well

[wing328]

CI failure not caused by this PR:

[ERROR] Command execution failed.
org.apache.commons.exec.ExecuteException: Process exited with an error: 1 (Exit value: 1)
	at org.apache.commons.exec.DefaultExecutor.executeInternal(DefaultExecutor.java:404)
	at org.apache.commons.exec.DefaultExecutor.execute(DefaultExecutor.java:166)
	at org.codehaus.mojo.exec.ExecMojo.executeCommandLine(ExecMojo.java:804)
	at org.codehaus.mojo.exec.ExecMojo.executeCommandLine(ExecMojo.java:751)
	at org.codehaus.mojo.exec.ExecMojo.execute(ExecMojo.java:313)
	at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:132)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:116)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:80)
	at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
	at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:120)
	at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:355)
	at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:155)
	at org.apache.maven.cli.MavenCli.execute(MavenCli.java:584)
	at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:216)
	at org.apache.maven.cli.MavenCli.main(MavenCli.java:160)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
	at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
	at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.6.0:exec (compile-test) on project HaskellServantServerTests: Command execution failed. Process exited with an error: 1 (Exit value: 1) -> [Help 1]

We will comment out the Haskell client test for the time being.