invoices: implement statelss invoice functinoality

[Roasbeef]

After the merge of #5810, we start to set the new metadata field in invoices, but don't yet do anything with them on the sender side. One thing this allows is for the reactor of an invoice to only actually commit state to the invoice once it's been fully paid, a.k.a "stateless" invoices.

In order to actually make this feature useful, we should draft up an initial version of stateless invoice interpretation within lnd itself. At a glance, we can likely re-use most of the keysend logic, which itself inserts an invoice into the DB just-in-time when the HTLC is received.

[TheBlueMatt]

You shouldn't need payment metadata's for this, fwiw. As long as you don't want additional user metadata (which they can always store out of band), you can use the scheme we use in LDK and just use the payment secret - lightningdevkit/rust-lightning#1177