FEATURE: Add a onebox_locale site setting. (#30655)

Following on from f369db5ae9, this change adds the ability to choose a custom locale to send to onebox providers.

If this setting is left blank, it will fall back to using default_locale.

app/models/onebox_locale_site_setting.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+# onebox_locale is just like any other locale setting, except it allows for an empty value,
+# which is used to indicate that the default_locale should be used for onebox_locale.
+class OneboxLocaleSiteSetting < LocaleSiteSetting
+  def self.valid_value?(val)
+    supported_locales.include?(val) || val == ""
+  end
+
+  @lock = Mutex.new
+end