Add custom HTTP headers for subscription updates

https://github.com/2dust/v2rayNG/pull/5856

V2rayNG/app/src/main/res/values/strings.xml

@@ -311,6 +311,7 @@
     <string name="sub_setting_remarks">remarks</string>
     <string name="sub_setting_url">Optional URL</string>
     <string name="sub_setting_user_agent">User Agent</string>
+    <string name="sub_setting_request_headers">Request Headers (JSON format)</string>
     <string name="sub_setting_filter">Remarks regular filter</string>
     <string name="sub_setting_enable">Enable update</string>
     <string name="sub_auto_update">Enable automatic update</string>