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)

app/finders/user_recent_events_finder.rb

@@ -49,9 +49,13 @@ def execute_single
   # rubocop: disable CodeReuse/ActiveRecord
   def execute_optimized_multi(users)
     Gitlab::Pagination::Keyset::InOperatorOptimization::QueryBuilder.new(
-      scope: Event.reorder(:id),
+      scope: Event.reorder(id: :desc),
       array_scope: User.select(:id).where(id: users),
-      array_mapping_scope: -> (author_id_expression) { Event.where(Event.arel_table[:author_id].eq(author_id_expression)) },
+      # Event model has a default scope { reorder(nil) }
+      # When a relation is rordered and used as a target when merging scope,
+      # its order takes a precedence and _overwrites_ the original scope's order.
+      # Thus we have to explicitly provide `reorder` for array_mapping_scope here.
+      array_mapping_scope: -> (author_id_expression) { Event.where(Event.arel_table[:author_id].eq(author_id_expression)).reorder(id: :desc) },
       finder_query: -> (id_expression) { Event.where(Event.arel_table[:id].eq(id_expression)) }
     )
     .execute