Add support for Cashu - Chaumian e-cash

[toneloc]

Cashu Nutshell has direct CLN support, which could be used with Stable Channels.

The requirements for using Stable Channels with Cashu would be that:

1. You'd be able to resize the position as you wish.
2. You know in advance by how much the fiat-denominated balance will change when making or receiving a bitcoin-denominated payment / invoice

Requirement 1 could happen when we implement Positions as in Issue #2

Requirement 2 would just need to feed that data when appropriate.

Thanks @callebtc

[toneloc]

Gonna revisit this issue this week.

[callebtc]

If I may add one more detail to 2), the change of the USD balance must also include the transaction fee in USD, which may already be the case in your implementation.

Can the resizing of the position be done by paying / receiving a lightning invoice?