feat: add default XBLOCK_HANDLER_TOKEN_KEYS

Description

This MR adds a default XBLOCK_HANDLER_TOKEN_KEYS populated with the current SECRET_KEY if any is set. Otherwise, generate a new SECRET_KEY. If the XBLOCK_HANDLER_TOKEN_KEYS were already set to any values, no changes will be made.

Supporting information

• https://github.com/openedx/edx-platform/blob/master/openedx/core/djangoapps/xblock/utils.py#L15-L89
• https://tasks.opencraft.com/browse/SE-5729

Testing instructions

Steps to test the changes:

1. Proofread
2. Checkout repo
3. Go to control dir
4. Execute ./grove prepare <INSTANCENAME>
5. Check that the desired setting is defined in the config.yml of the instance.

Dependencies

• tutor-contrib-grove!81 (merged)

Screenshots

N/A

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

N/A