Set invoice+bip21 memo based on referral key

[kilrau]

Substituting https://github.com/BoltzExchange/boltz-backend/issues/371, we want to make setting invoice memo and bip21 memo depending on referral key which solves the issues mentioned in #371 .

current referral keys -> stay as-is to not break any UIs boltz_webapp_desktop -> Boltz Swap boltz_webapp_mobile -> Boltz Swap new referral keys -> already got some custom wishes

Preservers privacy via Web App and gives integrations what they want. Everyone is finally happy.