USD-backed ecash using stablesats

[ngutech21]

• Adds new lightning backend that uses the stablesats gql-api provided by galoy (1 ecash token represents 1 usd cent)
• New endpoint GET /melt/:invoice to get a usd price of a ln-invoice

This is a proof of concept!

TODOs:

• better error handling
• use rust_gql crate
• figure out how to handle fees
• fix backwards compatibility with btc backed tokens. At the moment the wallet assumes that the mint always provides the GET /melt/:invoice endpoint