Use per-resource version to avoid hashing protos in xDS updates

xDS APIs today use hashes of incoming resource protos to compare against the existing resource version when determining if any updates needs to be made. This is expensive (we're measuring ~8% CPU in some workloads), since we need to flatten to text for Envoy-canonical proto hashing (protobuf doesn't do its own hashing).

It's also unnecessary when delivered resources are provided via delta xDS (or even SotW when we encapsulate in a Resource object). In these cases, we have per-resource versioning that can play the same role as the hash without any computational requirement.

Opening this issue to track potential optimization work here. I think the main value is for SotW+Resource wrapper, since delta xDS will be pushing the diffing to the server-side anyway, so there isn't a lot of saving.

FWIW, I think we had a thread on this ages ago but I can't spot it.