Provide signatures for webhook alerts

### Current Behavior:

Alerts / notifications sent via Webhook are neither authenticated nor signed in any way.
This makes it hard for receiving parties to verify whether a given notification was sent by DT.

### Proposed Behavior:

For the *Webhook* alert type, allow for an optional shared secret to be provided.
Before sending the webhook request, calculate an HMAC for the JSON payload, and include the resulting value in a request header (e.g. `X-Webhook-Signature`).

For reference, this is also how GitHub is doing it: https://docs.github.com/en/developers/webhooks-and-events/webhooks/securing-your-webhooks