Create a basic delete appointment request

[sr-gi]

Following the current version of BOLT13's draft, the user should be able to delete appointments from the tower.

The goal of deletion is to free space in ongoing subscriptions for new appointments. This would normally happen when a node has closed a channel and wants to reuse those appointment slots to cover other channels. Since the tower does not know about cooperative closes, it is left for the user to decide when to delete appointments and how.

The goal of this issue is to cover basic appointment deletion:

• The user picks an appointment they want to delete
• The user sends a request to the tower requesting for the deletion
• The tower deletes the appointment provided that the user has an associated appointment with the same locator
• The tower frees the necessary slots for the user

User authentication is performed over EC-recovery, so a signature must be provided by the user to delete the message. The should follow the same approach as for getting appointments.

PS: Build on top of branch 17-gatekeep-api if it has not been merged to master by the beginning of the hackathon.

[bjarnemagnussen]

I will try to tackle this.

[sr-gi]

Hey @bjarnemagnussen, I was thinking on picking this up where you left it. Would you mind creating a draft PR with the current version?