Rollup node: Allow to specify name of exported snapshot file
Context
The command
octez-smart-rollup-node snapshot export snap.archive
will create the snapshot in ./snap.archive file, while
octez-smart-rollup-node snapshot export
will create the snapshot in a file of the form ./snapshot-<address>-<level>.<history_mode>
This change makes the rollup node command more closely follow the octez node one and was requested by @chrispinnock for the etherlink snapshot service.
Edited by Alain Mebsout