FCM push through a proxy

[chibenwa]

What

In security constrained environment direct HTTP requests to the outside may be forbidden and HTTP calls may need to be proxied, which includes FCM calls done by the backend.

Thus we wishes to proxy Tmail backend calls through a proxy...

How

FCM driver allows for such options

In JVM properties this seems enough:

# com.google.api.client.http.javanet.NetHttpTransport L 82
com.google.api.client.should_use_proxy=true
# com.google.api.client.http.javanet.NetHttpTransport::defaultProxy
https.proxyHost=192.168.12.45
https.proxyPort=443

If needed use it.

Can be documented to ease future use?