feat(webhooks): streamline webhook response data

[mantariksh]

Problem

The webhook response data stored in the database has a couple of superfluous fields:

1. errorMessage is useless because it is always a standard axios error message, e.g. "Request failed with status code 400".
2. statusText is useless because it is just the standard HTTP status text, e.g. 'OK' for 200

Furthermore, fields that should always be present (webhook URL, signature, status code) are not marked as required in the database schema.

Solution

Remove the superfluous fields and mark the rest as required where appropriate. headers and data were not made required as they may be empty strings.

[karrui]

lgtm