Add merge-cert subcommand
This PR is a proposal for a new subcommand merge-cert which allows to merge together multiple revisions of a certificate.
This operation is useful e.g. if another use issued a certification signature that the user wants to merge into their certificate.
TODO:
- [ ]: Better error code for mismatching certificates
- [ ]: Consider also handling secret keys (merging a secret key with an updated revision of its certificate)
- [ ]: Provide guidance on how to do the merging (canonicalization)
- [ ]: Allow merging standalone signatures into a certificate