Implement lightning send/receive

[bonomat]

#11 upgrades to v8.
V8 comes with support for sending and receiving via lightning, see these e2e tests as example:

• https://github.com/arkade-os/rust-sdk/blob/1444706c10643116c05faddbbfb012da379847d0/e2e-tests/tests/boltz_reverse.rs#L17
• https://github.com/arkade-os/rust-sdk/blob/1444706c10643116c05faddbbfb012da379847d0/e2e-tests/tests/boltz_submarine.rs#L22