feat: add environmental expansions for exemption addresses

Closes #8 (closed)

This change allows environment variables to be expanded in exemption addresses.

Related to https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/5127

---

Environment Substitutions

This change allows environment variables will be expanded in exemption addresses in exemption files.

For example, given the following example:

exemptions:
- module.provisional_regional_r0.module.get.module.item.module.consul.aws_ebs_volume.gitlab["${TENANT_ID}-consul-1-logs"]
- module.provisional_regional_r0.module.get.module.item.module.consul.aws_ebs_volume.gitlab["${TENANT_ID}-consul-2-logs"]
- module.provisional_regional_r0.module.get.module.item.module.consul.aws_ebs_volume.gitlab["${TENANT_ID}-consul-3-logs"]

TENANT_ID will be replaced with the value of the environment variable TENANT_ID.

If the environment variable is not set, an empty string value will be used.

Additionally, the older approach to exemptions (configured through environment variables with date windows) has been removed.