gRPC implementation of PeerPicker

[desimone]

Would you accept a PR adding a gRPC implementation of PeerPicker into this repo or would you prefer that lived elsewhere?

Previously #53

[malikbenkirane]

@hsinhoyeh @xiang90 @bradfitz Anything new about this ?

Anyone nows if this is livings somewhere else ?

Also I've found https://pkg.go.dev/github.com/charithe/gcgrpcpool

Though, I'm not sure that this latter follows the same idea as in #53