FEATURE: use new site setting for onebox custom user agent. (#28045)

Previously, we couldn't change the user agent name dynamically for onebox requests. In this commit, a new hidden site setting `onebox_user_agent` is created to override the default user agent value specified in the [initializer](c333e9d6e6/config/initializers/100-onebox_options.rb (L15)).

Co-authored-by: Régis Hanol <regis@hanol.fr>

lib/onebox/helpers.rb

@@ -104,9 +104,7 @@ module Onebox
 
         headers ||= {}
 
-        if Onebox.options.user_agent && !headers["User-Agent"]
-          headers["User-Agent"] = Onebox.options.user_agent
-        end
+        headers["User-Agent"] ||= user_agent if user_agent
 
         request = Net::HTTP::Get.new(uri.request_uri, headers)
         start_time = Time.now
@@ -232,6 +230,12 @@ module Onebox
       end
     end
 
+    def self.user_agent
+      user_agent = SiteSetting.onebox_user_agent.presence || Onebox.options.user_agent
+      user_agent = "#{user_agent} v#{Discourse::VERSION::STRING}"
+      user_agent
+    end
+
     # Percent-encodes a URI string per RFC3986 - https://tools.ietf.org/html/rfc3986
     def self.uri_encode(url)
       return "" unless url