lsp_plugin: add htlc_accepted handler for no-mpp

Adds the service side (LSP) for a simple no-mpp trusted jit channel
opening. This is only an intermediate step, we are going to add support
for multiple htlcs.
This is experimental and can drain on-chain fees from the LSP if used in
public.

Signed-off-by: Peter Neuroth <pet.v.ne@gmail.com>

Author: nepet