Provide an option to suppress "Unknown library option" message in JavaClientCodegen

Description

When developing custom libraries by extending the JavaClientCodegen, it prints an error message for any libraries the parent class doesn't recognise when doing procesOpts.

Unknown library option (-l/--library): myjavaclient

This is misleading and we should be able to control whether want to display this or not from subclasses.

Swagger-codegen version

Both 2.x and 3.x

It comes from https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/JavaClientCodegen.java#L285 and https://github.com/swagger-api/swagger-codegen-generators/blob/master/src/main/java/io/swagger/codegen/v3/generators/java/JavaClientCodegen.java#L268

Steps to reproduce

1. Extend JavaClientCodegen and set custom library

public class MyJavaClientCodegen extends JavaClientCodegen {

    public MyJavaClientCodegen() {
        supportedLibraries.put("myjavaclient", "My Java client");
        setLibrary("myjavaclient");
    }

    @Override
    public String getName() {
        return "myjavaclient";
    }
}

2. Run the codegen (v2 pseudo example shown)

new DefaultGenerator()
    .opts(new CodegenConfigurator()
        .setInputSpec("/path/to/spec")
        .setOutputDir("/path/to/outputs")
        .setLang("myjavaclient")
        .toClientOptInput())
    .generate();

3. Observe that it prints Unknown library option (-l/--library): myjavaclient even though it generates the code successfully.

Suggest a fix/enhancement

I think one solution might be to expose a property (system/additional - please advise) that can be used to control whether this is logged. The default behaviour should be to continue logging unknown libraries. Let me know if you like this proposal and I will be more than happy to raise PRs for both v2 and v3 generators.

Start by reading JavaClientCodegen.java in the v2 and v3 generator paths identified in the issue, around the processOpts logic that reports unknown libraries. Check how subclasses register supportedLibraries and set a custom library. Done means custom libraries no longer produce the message when suppression is enabled, while unknown libraries still log it by default in both generator versions.