Allow to use description hash

[tnull]

A user demanded the ability to use description hash over setting a description.

We need to figure out a way that doesn't complicate our payments API too much.

[tnull]

Slipping, as we can't do this currently without pulling in a lot of code from upstream LDK. Blocked on lightningdevkit/rust-lightning#3371

[tnull]

Closing this as duplicate of #325.