Introduce ax snapshot

Introduce a client initiated snapshotting RPC and `ax snapshot` subcommand to initiate snapshots on Substrate.

Once a snapshot is taken, AX should return the snapshot_id and have a record for it in the event log.

Tweak the forking capability to accept a snapshot_id instead of a sequence number. Forks should copy the event log to a src_snapshot_id and should be able to resume from that snapshot.