Add a generic proto.Clone about protobuf HOT 1 OPEN

We discussed this internally and more or less aligned on

// CloneOf returns a deep copy of m. If the top-level message is invalid, 
// it returns an invalid message as well.
func CloneOf[M Message](m M) M {
        return proto.Clone(m).(M)
}

We decided to not move forward with it because at the time some of the tooling we use internally wasn't ready for generics. The remaining difficulty is that we are still supporting go 1.17, but we can work around that with build tags.

We're happy to accept a contribution. :)

Regarding Merge and Equal a separate proposal would be good.

from protobuf.