Add option to set anypay backend url in the settings

[pshenmic]

Issue

Users should be able to easily switch backend API url in the settings of the merchant app. It will allow merchant-app users to not rely on the official AnypayX backend and even use it with their own hosted backend.

Details

• Add an input or dialog in the settings that will allow you to set or switch backend urls
• User should be able to supply HTTPS and HTTP URL