Implement layer auto-discovery

[corrieriluca]

Goal

Allow Burrito to automatically detect Terraform/Terragrunt/OpenTofu layers in a repository, without manual/templated TerraformLayer configuration.

Sub-issues

• Spec & design (RFC)
• Controller implementation (repo scanning, resource creation)
• Unit and E2E tests
• User documentation

[michael-todorovic]

I'm very interested into this 😄
We're currently implementing self-service with Backstage and we're adding new tf layers. For now, we need to manually add the new layers so Burrito can kick in.
Could you please plan a webhook endpoint to force the refresh of TerraformRepository, then discovery of new layers? They would be in branches to begin so we get the tf plan