[services] Tunnelbroker - Add payload to APNs message to make it content-available

Summary:
This diff introduces changes to the Rust notification library to add an optional APNs push message payload.

To wake up the iOS app from the background we should send a message with the payload. In this case, a message will be with the content-available attribute, so we can do background information processing in the app.
Apple's related documentation, apple forum thread.

The message payload is optional in the library.

Related Linear task: ENG-1743

Test Plan:

1. Successfully built app without errors.
2. Device testing:

To test the APNs notification physical iOS device is required.

• Isolated testing can be done using the apns-ios-push-tester XCode project. To test it clone the apns-ios-push-tester and follow the instruction to set the certificate, run the app on the physical iOS device and get the device token.
• Comm app testing can be done using the apns certificate provided in 1Password.

Send the apns message using the following library call from C++:

#include "cxxbridge_code/src/lib.rs.h"
#include "rust/cxx.h
...
const std::string cert_path = argv[1];
const std::string cert_pass = argv[2];
const std::string topic = argv[3];
const std::string device_token = argv[4];
const std::string message_text = argv[5];
const std::string message_payload = argv[6];
const bool sandbox = true; // True if your certificate is for development.
sendNotifToAPNS(cert_path, cert_pass, topic, device_token, message_text, message_payload, sandbox);

The expected result is push message delivery to the device if the credentials are correct. Errors if some of the credentials are wrong.

Reviewers: jon, karol, tomek

Reviewed By: jon, tomek

Subscribers: ashoat, tomek, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D5083