feat: add GitHub webhook validation

Description

This MR implements GitHub Webhook secret validation. The webhooks are not validated at the moment, hence those who have the basic auth credentials could send any request. After merging this MR, the webhooks will be validated as well.

Supporting information

• https://tasks.opencraft.com/browse/SE-6063
• Getting the HTTP headers from env variables: https://github.com/openfaas/workshop/blob/master/lab4.md#inject-configuration-through-environmental-variables

Testing instructions

Steps to test the changes:

1. Check that https://github.com/open-craft/edx-platform/settings/hooks/434082134 has a webhook secret
2. Check that https://gitlab.com/opencraft/ops/grove-stage-digitalocean/-/hooks/29130938/edit has a webhook secret
3. Comment /grove sandbox update on https://github.com/open-craft/edx-platform/pull/583 and check for the PR comment that indicates the successful deployment start.

Dependencies

NA

Screenshots

NA

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated

Additional context

NA