Support rubocop plugin API

[nicolastemciuc]

Warnings

rubocop-rspec extension supports plugin, specify `plugins: rubocop-rspec` instead of `require: rubocop-rspec` in webauthn-ruby/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.

rubocop-rake extension supports plugin, specify `plugins: rubocop-rake` instead of `require: rubocop-rake` in webauthn-ruby/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.