feat: authenticate to GitLab Terraform module registry

Adds logic to authenticate to GitLab's inbuilt Terraform module registry in the init function.

Solves gitlab#346576 (closed)

References:

• Docs: Authenticate to the Terraform module registry
• Terraform's config discovery within .terraform.d: https://github.com/hashicorp/terraform/blob/df578afd7e34ae32345d86840edd05b4c05559e8/internal/command/cliconfig/cliconfig.go#L178