Add Key Superseded signature

The objective of this signature type is to ease the transition from v4 to v5 keys, and in the future to PQC certificates.

Some people might be interested in having more than one certificate to take advantage of the latest security features but still providing compatibility with legacy software. Using the Key Superseded signature this is possible, by having a soft revocation that older software will ignore, and newer software can actively use to look for a new key or warn that a superseded key is being used.