[feature request] Dedicated onchain wallet for PeerSwap

Moving the discussion from #88.

Idea:
Have a separate pool of funds used for PeerSwap, different from the CLN / LND onchain wallet on the same instance.
The LN wallet is already used for:
* funding channels
* receiving funds from closed channels (force closes always return to the internal wallet).
* CLN: liquidity ads / dual funded channels

The ability to keep the funds used for different purposes would be good for privacy and could also simplify the accounting.

Possibly related: https://github.com/GaloyMoney/charts/issues/1208