PKCS11 support

[scj643]

Adding support for PKCS11

Still need to work out how to actually reference the key objects and generate them.

I am currently only testing with a Yubikey but SoftHSMv2 could also be tested against.

Checklist

• Implement key management.
• Define where to store keys on the device.
• Store key metadata.
• Retrieve signer based on metadata.
• Write tests.
• Add documentation on using PKCS11

(More to be added)

[scj643]

This should resolve part of #84

[Foxboron]

Generally stuff looks fine. Please don't feel too constrained by how the file and tpm backend is done and just get stuff to work through hacks and weird stuff to get going :) Then we can work out the parts as we go along.