amount_btc vs amount_sats

[Amperstrand]

Mostly, fiat_link uses amount_sats, but in some places, the amount_btc is used.

For consistancy, I think it would be best if we always used the same everywhere: either bitcoin or sats

BTCPayserver uses amount in bitcoin as a string.

lightning-cli uses sats o msats.

Fiatlink should choose one and be consistent across all api calls.

[aljazceru]

@Amperstrand thanks for flagging the issues, it was an oversight during the review process

all amounts are now unified to sats, i've updated the documentation, swagger and reexported the clients