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/helpers/application_settings_helper.rb

@@ -144,36 +144,39 @@ def repository_storages_options_json
   end
 
   def external_authorization_description
-    _("If enabled, access to projects will be validated on an external service"\
+    s_("ExternalAuthorization|Access to projects is validated on an external service"\
         " using their classification label.")
   end
 
   def external_authorization_timeout_help_text
-    _("Time in seconds GitLab will wait for a response from the external "\
-        "service. When the service does not respond in time, access will be "\
-        "denied.")
+    s_("ExternalAuthorization|Period GitLab waits for a response from the external "\
+        "service. If there is no response, access is denied. Default: 0.5 seconds.")
   end
 
   def external_authorization_url_help_text
-    _("When leaving the URL blank, classification labels can still be "\
-        "specified without disabling cross project features or performing "\
-        "external authorization checks.")
+    s_("ExternalAuthorization|URL to which the projects make authorization requests. If the URL is blank, cross-project "\
+      "features are available and can still specify classification "\
+      "labels for projects.")
   end
 
   def external_authorization_client_certificate_help_text
-    _("The X509 Certificate to use when mutual TLS is required to communicate "\
-        "with the external authorization service. If left blank, the server "\
-        "certificate is still validated when accessing over HTTPS.")
+    s_("ExternalAuthorization|Certificate used to authenticate with the external authorization service. "\
+        "If blank, the server certificate is validated when accessing over HTTPS.")
   end
 
   def external_authorization_client_key_help_text
-    _("The private key to use when a client certificate is provided. This value "\
-        "is encrypted at rest.")
+    s_("ExternalAuthorization|Private key of client authentication certificate. "\
+        "Encrypted when stored.")
   end
 
   def external_authorization_client_pass_help_text
-    _("The passphrase required to decrypt the private key. This is optional "\
-        "and the value is encrypted at rest.")
+    s_("ExternalAuthorization|Passphrase required to decrypt the private key. "\
+        "Encrypted when stored.")
+  end
+
+  def external_authorization_client_url_help_text
+    s_("ExternalAuthorization|Classification label to use when requesting authorization if no specific "\
+      " label is defined on the project.")
   end
 
   def sidekiq_job_limiter_mode_help_text