RFE: Allow hkp keyservers to supply additional information to the user

[ikaneshiro]

Some hkp key server implementations my want to supply users with additional information upon key submisstion. E.g. have a user verify the identity supplied in a public key via an out of band process.

This was first proposed by these changes: https://github.com/hpcng/singularity/pull/5464/files/84c11e21820d976499085545a8b789404505f345#diff-89ea8388f9af37fc57250aa45f4d2991ce6c5b7dd4a53b63f543bb4a669dc878R1122 and subsequently pushed out to an RFE in the key server client we use: sylabs/scs-key-client#47

Either approaches should be sufficient in my measurement.

[DrDaveD]

Replaced by apptainer/apptainer#20