Replace Blink with Mint WebSocket connection

[Egge21M]

Feature Request

Is your feature request related to a problem? Please describe.

In order for npubcash to work properly it needs to know when invoices generated through LNURL get paid. Because polling mints is unfeasible, npubcash uses Blink to wrap each invoice and then uses Blink's WebHooks API to get notified.

While this works flawlessly, we should strive to reduce third party dependencies...

Describe the solution you'd like

Because polling is unfeasible we need a way for a mint to actively notify npubcash whenever an invoice has been paid. Mint WebSockets (cashubtc/nuts#98) would solve this issue. Once this is implemented in Mints we can savely transition to WebSockets and remove Blink API completely.