reactive: Add a new `define_command` DSL method for processors

This adds a new Command concern that can be used to define that a processor reacts to a particular command directed at the @gitlab-bot.

The concern can be enabled in a processor with on form of the following:

define_command name: 'command_name'

# A regex can be defined to detect command arguments
define_command name: 'command_name', args_regex: /\w+/

This is a refactoring prior to actually implement #767 (closed).

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Week-in-review

Related to #767 (closed).