Opt-in durability instead of Pause vs Suspend API

Instead of explicitly requesting Pause vs Suspend, I think applications should request `Suspend` with an argument specifying their tolerance, naming is hard but `Durable: true` or something like that, opt-in.

When applications do not specify, substrate can intelligently handle suspension, e.g. in the future it might:
- Suspend to local disk
- Decide not to suspend immediately due to low load
- Automatically move snapshots between nodes p2p to rebalance
- Decide to move the snapshot to cloud storage
- Live migrate sandboxes

Only if you have a hard need for an immediate backup, then we should explicitly move the snapshot to cloud storage (S3, GCS, ...).

I think the current Pause vs Suspend inverts this and doesn't give a lot of room to implement things like automatically moving snapshots between machines or live-migrate microVMs

See related discussion in https://github.com/agent-substrate/substrate/issues/660

And previously https://github.com/agent-substrate/substrate/issues/119