Add 'not compressed' bit to LZ4 fragmented compressor so it can skip compression/decompression of small items

[mykaul]

Here's an example of a very inefficient compression - 29 bytes (incl. 4 bytes length field (length = 25), before compressed content), translate to 4 bytes of the compressed length = 21 bytes of compressed data, which end up decompressed into ... 21 bytes of data. We've gained practically nothing here.

We could probably use another bit (just like last_chunk_flag - https://github.com/scylladb/seastar/blob/665fed057d6ce08de4a78659403203bcce79b6cb/src/rpc/lz4_fragmented_compressor.cc#L61C27-L61C42 ) in order to let the receiver know we did NOT compress this blob, as it was worthless.
Removing another bit from the compressed content 4 bytes field (leaving 30 bits 'only') seem OK to me, but of course will need to be negotiated between sides.

[mykaul]

CC @michoecho