Enforce single device access on VSS

[TonyGiorgio]

I've started to try to enforce single device access at a websocket level here: https://github.com/MutinyWallet/websocket-proxy/pull/2

However, I think it's a hack because it relies on the WSS proxy which is mostly a thing for just the LN node part. I think we should add an idempotency key to VSS.

This can either be in the form of another variable to the VSS parameters (easier and probably safer to move to first) and then eventually add WSS to VSS where it is enforced too.

This is kind of fedimint related since we're starting to rely on the LN node part less.

[benthecarman]

I was thinking that it is better on the websocket proxy because we don't need to enforce single device expect for lightning.

[TonyGiorgio]

We absolutely need to for ecash.