[Filestore] Send ReadData/WriteData buffers as an external payload

[antonmyagkov]

The tablet currently sends ReadDataResponse with the data buffer embedded inside the protobuf message:
https://github.com/ydb-platform/nbs/blob/main/cloud/filestore/libs/storage/tablet/tablet_actor_readdata.cpp#L1101

This approach causes several issues:

1. Parsing the buffer from the protobuf message on the filestore-vhost side is expensive.
2. The interconnect does not support transmitting protobuf messages over XDC/RDMA.

Instead, the data buffer can be sent as a message payload. This avoids the protobuf parsing overhead and enables XDC/RDMA transport support.