Accept maintainer_note parameter in REST runner registration

What does this MR do and why?

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This MR adds support for taking in a maintainer_note field in the runner registration endpoint. A gitlab/gitlab-runner MR will soon take advantage from this addition so as to allow an admin to specify notes to be associated with the new runner.

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Part of #348299 (closed)

lib/api/ci/runner.rb

@@ -15,6 +15,7 @@ class Runner < ::API::Base
         params do
           requires :token, type: String, desc: 'Registration token'
           optional :description, type: String, desc: %q(Runner's description)
+          optional :maintainer_note, type: String, desc: %q(Runner's maintainer notes)
           optional :info, type: Hash, desc: %q(Runner's metadata)
           optional :active, type: Boolean, desc: 'Should Runner be active'
           optional :locked, type: Boolean, desc: 'Should Runner be locked for current project'
@@ -25,7 +26,7 @@ class Runner < ::API::Base
           optional :maximum_timeout, type: Integer, desc: 'Maximum timeout set when this Runner will handle the job'
         end
         post '/', feature_category: :runner do
-          attributes = attributes_for_keys([:description, :active, :locked, :run_untagged, :tag_list, :access_level, :maximum_timeout])
+          attributes = attributes_for_keys(%i[description maintainer_note active locked run_untagged tag_list access_level maximum_timeout])
             .merge(get_runner_details_from_request)
 
           @runner = ::Ci::RegisterRunnerService.new.execute(params[:token], attributes)