Add proper e2e testing to the repository to ensure its components work

[Racer159]

Describe what should be investigated or refactored

Currently there isn't an easy way to test the sigstore stack without also deploying GitLab alongside it - once #3 is completed however we will have a way to easily test with only Keycloak (which is part of uds-core)

Links to any relevant code

https://github.com/defenseunicorns/uds-package-sigstore/blob/main/src/fulcio/values/values.yaml#L23

Additional context

This will also require changes to uds-core to support this properly: defenseunicorns/uds-core#509