Android push notifications with PassWallet

[mandyJ]

This PR adds support for Android push notifications using PassWallet.

It adds the registration endpoint called by PassWallet;
Adds a migration to add a pushServiceUrl column to the Devices table;
Registers an Android device with its pushServiceUrl. The presence of a pushServiceUrl indicates if the device is Android vs iOS.

If iOS, a library like apnotic can be used to send an Apple Push Notification, as demonstrated here

if Android, a POST request can be made to the pushServiceUrl as laid out by PassWallet documentation.

Why PassWallet?
While coorasse/passkit defaults to WalletPasses to open the Apple wallet format (.pkpass) on Android, I have found that WalletPasses does not support the multiple pass format (.pkpasses), but PassWallet does.

PassWallet supports adding passes by adding from file storage on the mobile device, which has made development a lot easier; PassWallet appears to be more feature rich in general compared to WalletPasses.

I also found the PassWallet developer documentation to be more descriptive: https://github.com/Kwiket/passwallet
compared to WalletPasses: https://walletpasses.io/developer/

[annaswims]

In the previous version, no changes are made to the db a device is found. Now, the device is being updated every time.

Maybe like this to back fill missing push_service_url?

          device = Passkit::Device.find_or_create_by!(identifier: params[:device_id]) do |d| 
          d.push_service_url = push_service_url 
          d.push_token = push_token 
          end
          
          device.update(push_service_url: push_service_url) if device.push_service_url.blank?

[annaswims]

This looks fairly atypical. Can you not acccess it with params["pushServiceUrl"]?