When sending nars over the wire to the nix-daemon, nix protocol versions
>= 1.23 use this framing protocol.
This change implements an AsyncRead for this protocol, to be used in
AddToStoreNar and any other operations when necessary.
Change-Id: I5f7972fe1c9ea145780bf449321bd3efeb833d18
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12814
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>