Allow Safeguards to be added and/or overridden outside of core

[jslucas]

Allow consumers to define their own safeguards and maintain existing behavior for backwards compatibility.

• Updates Safeguard to use a registry
  Similar to how RuboCop handles Cops from itself, gems, plugins, and local repos. Safeguard.run executes each registered safeguard.
• Adds Safeguard::Base class
  Consumers can create new safeguards by inheriting this class. Safeguards are added to the registry through inherited.
• Adds Safeguard::Deprecated class
  Just a marker for the existing core-defined safeguards. The idea is that these would eventually get deleted after the adapters are updated to implement their own.
• Enables "overriding" of core-defined safeguards
  If a safeguard is registered by the same name as a deprecated safeguard it will be executed instead.

This will allow the resolution issues like #659 and #684 by enabling adapters (or users) to define their own safeguards.

For example, to resolve #684 database_cleaner-active_record can implement its own AllowedUrl to override the existing behavior.

Simplified example:

# config/database.yml
test:
  adapter: postgresql
  host: postgres
  port: 5432
  database: myapp_test
  username: dbuser
  password: dbpassword
  pool: 5
  encoding: unicode

module DatabaseCleaner
  module ActiveRecord
    module Safeguards
      class AllowedUrl < DatabaseCleaner::Safeguard::Base
        def run
          return if skip?
          raise Error::UrlNotAllowed if database_url_not_allowed?
        end

        private

          def database_url_not_allowed?
            !DatabaseCleaner.url_allowlist.any? {|allowed| allowed === database_url }
          end  
         
         # ...
         
          def database_url
            return ENV['DATABASE_URL'] unless ENV['DATABASE_URL'].nil? || ENV['DATABASE_URL'].strip.empty?

            # ...build a connection url from ::ActiveRecord::Base.connection_db_config
            # => "postgresql://dbuser:dbpassword@postgres:5432/myapp_test=5&encoding=unicode"
          end
      end
    end
  end
end