Add granular webhooks

[Thinkscape]

Why

The original implementation would call the webhooks for both success and failure. Without checking the contents of the JSON body a health check service would be unable to distinguish between those two.

Typical healthcheck provider would expect different endpoints for success, start, progress and failure (if the latter is are supported at all).

Here's one example: https://healthchecks.io/docs/signaling_failures/

What

This extends the webhook support to be friendlier with dead-man-switch health check services.

It adds 3 more optional env vars and will call the specified endpoints as needed.

env var	description
WEBHOOK_ERROR_URL	URL to be called in case backup fails. Default disabled.
WEBHOOK_PRE_BACKUP_URL	URL to be called when backup starts. Default disabled.
WEBHOOK_POST_BACKUP_URL	URL to be called when backup completes successfully. Default disabled.

Backwards compatibility

The original WEBHOOK_URL and its behaviour is untouched, so this change is 100% backwards compatible.

[prodrigestivill]

Awesome, thank you for your work.

I just simplified both cases as a single one, just as style change.
In any case, great work.

[Thinkscape]

Legend. Thanks!