feat: add metadata field to make_invoice command

[rolznz]

This enables apps to easily attach metadata to invoices rather than requiring an external database. This can currently be done by encoding data in the description field which will be included in the invoice itself, but there are a few downsides:

• it makes the invoice QR code harder to scan
• user doesn't understand the encoded content which should actually be a description
• description is limited to 639 bytes and then gets switched to a description hash, which can break apps

TODOs:

• add to JS SDK - feat: add metadata field to make_invoice js-sdk#232
• update Bitcoin Connect
• test with Paper Scissors Rock game - feat: use make_invoice metadata instead of encoding data into invoice description rolznz/paper-scissors-hodl#1
• review max metadata size (currently 2KB encoded)
• suggest change to NIP