GCPSigner: test all of the keytypes

[jku]

Current test code for the Google Cloud Platform Signer only uses one ecdsa-sha2-nistp256 key.

GCPSigner supports more keys: this has not really been tested though: I just wrote the mappings without creating anything except the two ecdsa keys. It would make sense to test these all. I'm not sure about testing all keys on every test run* but at least a manual test of all of them would make sense.

This requires:

• Creating the keys on KMS using a keyscheme-based naming scheme, and configuring them for use from this project (I am currently an admin on the google cloud project but I could add someone else for this too)
• building the gcp_ids in the test with knowledge of that naming scheme

*) I would rather keep the signing operation count fairly low on the Google Cloud KMS: not because it's expensive but because it allows me to easily set usage limits on those operations.