Support Splicing in LSPS2

[TheBlueMatt]

We shouldn't open a new channel with a client if we already have one. This may be a simple doc + ldk-node update, or there could be more, not sure.

[tnull]

This may be a simple doc + ldk-node update, or there could be more, not sure.

It is, especially since anything beyond just using splicing when you'd usually use a new channel wouldn't work out, cf the discussion over at lightning/blips#65. I'm going ahead and move this issue over to LDK Node.

[Jolah1]

I'd like to work on this.

My approach is to check for an existing channel with their_network_key in the OpenChannel service event handler, and splice into it instead of opening a new one when one exists.

I'll also update the LSPS2ServiceConfig docs to reflect the new behavior.

[tnull]

No, unfortunately this is currently still blocked on the bLIPs PR linked above, which I will take over soon to make some progress there.

[Jolah1]

Alright then!