Clarify preimage format / encoding

[philippgille]

Currently the value in the "x-preimage" header is required to be Base64, because that's the format that's used in listed invoices when executing lncli listinvoices. Not only for preimages, but also for their hashes.

But on the other hand the LightningClient's method LookupInvoice(...) expects a hex-encoded payment hash.

That's why in our package, when receiving a request with a correct x-preimage header, we need to:

1. Base64 Decode the preimage
2. Sha256 the previous value
3. Hex the previous value
4. Send it to lnd

So:

1. Figure out which format is best for using in the header (Plain preimage, Base64 or hex)
2. Change our implementation in case it's not Base64
3. Document which format is expected

[philippgille]

Closed by b1d5c04