Add multistore support

[BitcoinMitchell]

Is your feature request related to a problem? Please describe.
Currently the module does not support Prestashop's multistore feature. This means that once you've connected the module to a BTCPay Server, that server (and selected store) will always be used.

Describe the solution you'd like
Configuration related to the module should be stored in a multistore compatible way.

Sketch/Image/Wireframe/Mockup
A nice detailed write-up about how to develop multistore modules can be found here: How to develop MultiStore compatible module in Prestashop.