[Feature request] Allow to set memo when paying a lightning invoice

[chloehjung15]

It would be great if we can set the memo to show in lnbits when we are paying a lightning invoice using lnurl. At the moment, I am using /api/v1/payments call to pay for an invoice but I can see that the "memo" value gets ignored in that api call if the "out" is true. It seems to use the description from the invoice but I am hoping to pass in the separate memo value in the api call so it's easy for us to tell what this payment was for in lnbits.