Support Webhook

[gerynugrh]

RFC

To Clarify:

1. When will we send webhook? For each email being received by addy, or for any outgoing email as well?
2. Depending on (1) should we do both incoming and outgoing in a single PR or should we split it?
3. Is there an existing design for the webhook payload?
4. Should we be sending an email for every failed webhook? This would be quite spammy, I guess it's okay if we limit to only send it once a day? Benchmarking from other services, they usually have circuit breaker to stop the service from sending any webhook if it keeps responding with failure

Todo:

• Webhook sending portion
• Use spatie for signature and flow control (see more)

To consider:

1. Which queue should we use, is internal queue okay or should we opt for external one, e.g Redis? I notice we already have an existing queue backed by redis, I will follow the existing implementation
2. Should we allow both http and https or only https?