feat: shed: incoming block-sub chainwatch tool

[magik6k]

This just imports an extremely stripped down version of the lotus-chainwatch tool from testnet days, which makes it possible to look at block graphs including blocks which weren't synced.

Usage:

# start the daemon, let it run for a while
$ ./lotus-shed chainwatch run
# in another terminal after chain is synced
$ ./lotus-chainwatch-simple dot > blocks.dot 2057234 2000 && dot blocks.dot -Tsvg -Grankdir=TB > blocks.svg
# (first number is the lower epoch (oldest tipset to see), second is the number of tipsets to get)