Adds support for PKCS#11 v2.20 and/or KMIP 1.2 compatible HSMs.
Includes backward compatible extensions to the krill.conf syntax.
Includes HSM related developer docs.
Excludes HSM specific API (REST, krillc, Prometheus) or UI extensions.
Support multiple signers of different types behind a HSM feature flag, and support in principle selecting which signer to use for which purpose. (#539)