Multi-LSP support

[tnull]

Currently, we only allow to set a single LSP for each protocol (i.e., bLIP-51/bLIP-52). We however got users requests for the ability to configure multiple LSPs at the same time.

While the technical change to do this is trivial, we'll need to figure out a good API design to allow for this. In case of bLIP-52 / LSPS2 we could actually request fee params from all LSPs in parallel and choose the cheapest one. For bLIP-51/LSPS1 it's however not quite so clear how we can handle multiple LSPs in parallel in the API.