LSPS1 client support

[kaloudis]

Now that LSPS1 is out in the wild, we'd like to add client support to ZEUS, at least for our LND based backends.

We have a reference implementation client available in bos (Balance of Satoshis) here: https://github.com/alexbosworth/balanceofsatoshis/blob/master/lsp/lsps1_client.js

According to the author of the implementation, Nitesh, it only requires API calls to SendCustomMessage and SubscribeToCustomMessage

This would be a great way for users to purchase channels in advance from OLYMPUS and other LSPs within the app.

Spec: https://github.com/BitcoinAndLightningLayerSpecs/lsp/blob/main/LSPS1/README.md

Public chan offerings: https://sparkseer.space/chanoffers

[kaloudis]

#2097