feat: support selfhosted spacelift urls (#5)

[jstrese]

This change addresses #5.

With this change we introduce the spacelift_url variable. The default value is app.spacelift.io. When using the default value, no changes should be noticed from previous versions.

When spacelift_url is set to the non-default value, a new policy file will be applied (assets/policy-custom.rego):

• The templatefile function will replace ${spacelift_url} in the OPA file with the value of var.spacelift_url
• URLs in this custom OPA file use the following formats
  • ${spacelift_url}/module/
  • ${spacelift_url}/stack/

Testing
Two new tests have been introduced

1. Test setting spacelift_url to a non-default value
2. Negative test for an invalid custom spacelift_url

Other Changes

1. The hard-coded space_id in tests was changed to var.spacelift_space_id. This allows non-spacelifters to test.