Google KSM Signer implementation for pyHanko
In order to sign PDFs using a Google KMS key, you need to do the following:

1. Create a key using Terraform (main.tf) or manually
2. Generate a CSR using the created signing key by using the generate_csr.py (work in progress, clean up required!)
3. Submit the CSR to a CA of your choice and receive a certificate
4. Signs PDFs using the Cloud KMS private key and the locally stored certificate (not implemented yet!)