Gradle: publish protobufs in 'hapi/hedera-protobufs' as 'hedera-protobuf-java-api'

[jjohannes]

This replaces the Maven based publishing that currently lives in https://github.com/hashgraph/hedera-protobufs-java.

The published Jars end up here: https://repo1.maven.org/maven2/com/hedera/hashgraph/hedera-protobuf-java-api/

The new mechanism has to produce a Jar with the same structure as the already published ones.