[Filestore] Support batch xattrs operations in grpc API

Right now xattrs operations are either for a single xattrs name or return all names without values. This is not very convenient if client needs to get (and probably set) multiple/all xattrs values.

At very least we want to be able to get all xattrs key-value pairs in a single request.

Ideally we also need a way to set multiple xattrs in a single call or provide them during node creation