HTTP/2 Support

[emindeniz99]

Thanks for stopping by to let us know something could be better!

Is your feature request related to a problem? Please describe.
Firebase Java Admin SDK Batch Send HTTP/1 issues
firebase/firebase-admin-java#834
firebase/firebase-admin-java#849

Describe the solution you'd like
What you want to happen.
Reactor netty transport for http2 requests
https://github.com/emindeniz99/google-http-java-client

Describe alternatives you've considered
I have tried different http transport layers for fcm library such as netty-reactor, java11-httpclient, springwebflux
main...emindeniz99:google-http-java-client:main
https://github.com/emindeniz99/google-http-java-client

note: java 11 http client is not good for http2 requests, you should create multiple client for same target host in order to send multiple requests, it creates only one connection per host. I experienced the issue :D so I switched to netty-reactor

Additional context
I am not sure which approach is best for creating a reactive or async library (CompletableFuture, Mono ... as return type).
pull or push based stream etc.
Google http client is used by many google developed libraries, http2 is essential for nowadays for the performance and optimization of resources. Can you consider the issue?