Add Rails/EnumKeywordArgs cop

[maxprokopiev]

This PR adds a new cop called Rails/EnumKeywordArgs.

This cop checks for the use of deprecated keyword arguments in enums. See more in #1238

Style guide PR: rubocop/rails-style-guide#356

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[maxprokopiev]

see rubocop/rails-style-guide#356 (comment)

[ghiculescu]

see rubocop/rails-style-guide#356 (comment)

A linter would still be helpful if it helps people autocorrect so they can migrate to the new API automatically.

[maxprokopiev]

reopening this one since there seem to be some demand to have a linter, but not an entry to the style guide