FEATURE: Show available interpolation keys when overriding translations (#22220)

This is the first of a number of PRs aimed at helping admins manage their translation overrides. It simply adds a list of available interpolation keys below the input field when editing an override.

It also includes custom interpolation key.

app/serializers/site_text_serializer.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class SiteTextSerializer < ApplicationSerializer
-  attributes :id, :value, :overridden?, :can_revert?
+  attributes :id, :value, :interpolation_keys, :has_interpolation_keys?, :overridden?, :can_revert?
 
   def id
     object[:id]
@@ -11,6 +11,14 @@ class SiteTextSerializer < ApplicationSerializer
     object[:value]
   end
 
+  def interpolation_keys
+    object[:interpolation_keys]
+  end
+
+  def has_interpolation_keys?
+    object[:interpolation_keys].present?
+  end
+
   def overridden?
     if options[:overridden_keys]
       options[:overridden_keys].include?(object[:id])