[bug][dart] Allow derived generators to call DefaultCodegen#processOpts

DefaultCodegen#procesOpts sets up quite a bit which may be expected by users (template directory overrides, git user/repo, etc). The way the dart-dio and dart-jaguar processOpts methods were written, these things did not work. This makes it work.

cc all dart contributors for awareness:

• Dart: @yissachar
• Dart (refactor): @joernahrens
• Dart 2: @swipesight
• Dart (Jaguar): @jaumard
• Dart (Dio): @athornz

To repro the bug, copy the dart README.mustache (for dart-dio or dart-jaguar) to some directory called template, make some changes to the README, for example add to the top:

# Testing https://{{gitHost}}/{{gitUserId}}/{{gitRepoId}}.git

Generate with:

openapi-generator generate --generator-name dart-dio -t $(pwd)/template --input-spec openapi.yaml --output client/

We'd expect this README to be generated, but as of current master it is not. This was reported as a question by a member of the community in Slack. I thought it was going to be a quick assist, but turned out to be an issue. There's no associated GitHub issue, although I'd image this might resolve other issues with dart-dio and dart-jaguar because there's an attempted workaround in the extended DartClientCodegen which leads me to believe there was a known confusion with templateDir:

        final Object isSupportDart2 = additionalProperties.get(SUPPORT_DART2);
        if (Boolean.FALSE.equals(isSupportDart2) || (isSupportDart2 instanceof String && !Boolean.parseBoolean((String) isSupportDart2))) {
            // dart 1.x
            LOGGER.info("Dart version: 1.x");
            supportingFiles.add(new SupportingFile("analysis_options.mustache", "", ".analysis_options"));
        } else {
            // dart 2.x
            LOGGER.info("Dart version: 2.x");
            // check to not overwrite a custom templateDir
            if (templateDir == null) {
                embeddedTemplateDir = templateDir = "dart2";
            }
        }

I don't follow why the if final if condition in the above snippet exists. Maybe someone familiar with these generators could add more of a comment (why are we setting dirs here when they should be handled in constructor and super.processOpts?)

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