This PR adds a parameter to startup, --synctarget. The synctarget flag is a developer-flag, that can be useful in some scenarios as a replacement for a CL node. It defines a fixed block sync target:
geth --syncmode=full --synctarget=./block_15816882.hex_rlp
The --synctarget is only made available during syncmode=full
rjl493456442