lsp_plugin: add lsps-jitchannel handler

Adds the full roundtrip to request a jit channel from the LSP. It
approves the jit scid returned by the LSP and returns the invoice with
the corresponding route-hint.

Changelog-Added Experimental support for LSPS2 no-MPP,
Lsps-trusts-client mode. See
https://github.com/lightning/blips/blob/master/blip-0052.md for further
details.

Signed-off-by: Peter Neuroth <[email protected]>

Author: nepet