Allow Java source formatting mvn formatter:format

[nickcmaynard]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch 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)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

More work towards our code style standards aim #3972.

Uses Eclipse formatting tools with a configuration matching Google's style guide (plus our customisations).

This brings in an appropriately configured Java formatter maven plugin, but doesn't run it as part of the build. To manually run it:
mvn formatter:format

Having done this once, we fix ~2200 code style warnings.

➜  ~/Documents/Personal/swagger-codegen$ mvn clean compile | wc -l
    3809
➜  ~/Documents/Personal/swagger-codegen$ mvn formatter:format | wc -l
      80
➜  ~/Documents/Personal/swagger-codegen$ mvn clean compile | wc -l
    1660

I haven't included the results of a run in this PR, as it would generate lots of merge conflicts... I'd suggest whoever merges this PR does a once-off formatter run.

[wing328]

@nickcmaynard thanks for the PR. I'll run mvn formatter:format later as there're some PRs about refactoring DefaultCodegen.

[nickcmaynard]

@wing328 Do you think the codebase is ready for a formatter run yet?

[wing328]

@nickcmaynard I'll do it after merging latest enhancements in the master to 2.3.0 first (there are lots of conflicts I need to resolve after removing the licenses from the templates)

[nickcmaynard]

Fair - this will cause some conflicts. Just making sure it doesn't get buried under all the other stuff you're doing!

[wing328]

Yup, I've created #4295 for tracking.