chore: use `--flashblocks.addr` for listening socket

[0x416e746f6e]

currently --flashblocks.enabled accepts socket address, which is confusing (and is not aligned with the way underlying reth does these things).

please change to any of the following:

• --flashblocks.enabled is a toggle, --flashblocks.addr accepts socket (with the default at 0.0.0.0:1111)
• no --flashblocks.enabled, --flashblocks.addr w/o default, specifying --flashblocks.addr enables it
• same as above, just with --flashblocks instead of --flashblocks.addr