Investigate direct protoc artifact dependency for maven plugin

[johanandren]

Right now the protoc-jar library does it's own resolution which I think side steps maven proxy config, and the jar also includes a few dated protoc exes, if we could depend more directly on protoc exes from maven central and use those that would make it easier to use akka-grpc maven behind corporate proxies/artifact repo mirrors.

[johanandren]

A bit of incomplete investigation in https://github.com/akka/akka-grpc/tree/wip-maven-resolution-protoc, seems doable but a bit of work