Add Boltz webhook handling

[dangeross]

This PR adds SwapUpdatedPayload to handle Boltz webhook requests.

@roeierez Is it worth filtering out some of the status updates on the notifier, or let the Notification Plugin decide which to ignore? I imagine SwapCreated, InvoiceSet and potentially Mempool statuses could be ignored.

[roeierez]

@dangeross yes we must filter those otherwise user will get lots of uselss push notifications.
I suggest also to open an issue in boltz repo to add the filter in the registration request. IMO it works well if we define only status updates that we are interested on.

[erdemyerebasmaz]

Have one comment about a value on SwapUpdatedPayload.