Accept maintainer_note parameter in REST runner registration

What does this MR do and why?

Describe in detail what your merge request does and why.

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/views/sherlock/transactions/show.html.haml

@@ -8,13 +8,11 @@
   %li
     %a{ href: "#tab-queries", data: { toggle: "tab" } }
       = t('sherlock.queries')
-      %span.badge.badge-pill
-        #{@transaction.queries.length}
+      = gl_badge_tag @transaction.queries.length.to_s, variant: :pill
   %li
     %a{ href: "#tab-file-samples", data: { toggle: "tab" } }
       = t('sherlock.file_samples')
-      %span.badge.badge-pill
-        #{@transaction.file_samples.length}
+      = gl_badge_tag @transaction.file_samples.length.to_s, variant: :pill
 
 .row-content-block
   .float-right