Add Reverse Swap notification

[dangeross]

This PR adds background processing of the reverse swap flow using webhooks:

• Register a reverse swap lockup address for notifications
• Add process_reverse_swap method to handle processing an individual reverse swap by address
• Adapt notification plugin handling for the push notification
• Add ReverseSwapUpdated event

Fixes #924

[roeierez]

Looks good! dropped two comments.

[roeierez]

I think it would be nice to be able to recognize whether it is a swap or reverse swap without trying both.
If we pass a query param app_data in the webhook url it returns back in the notification payload.
See here where it is grapped from the query param: https://github.com/breez/notify/blob/main/http/router.go#L40
And here how it is populated into the payload: https://github.com/breez/notify/blob/main/breezsdk/init.go#L43
Perhaps we can use it here to identify if it is a swap or reverse swap?

[dangeross]

Yeah, I thought that too, but the redeemSwap first checks there is a swap for that address so it's pretty inexpensive.

What if the application or NDS is already using the app_data param in someway? We would override it and potentially break NDS/app logic.

[roeierez]

That's a good argument. I am ok then to leave this inexpensive check.

[erdemyerebasmaz]

Should we rename the notification channel & notification channel group variable as well?

[erdemyerebasmaz]

LGTM

[roeierez]

LGTM