Add a method of reusing a client certificate

[davep]

Right now, if a site requests a certificate and one doesn't exist, Rogallo simply goes into a certificate-creation mode. It might be useful to add the ability to pick an existing certificate and reuse it.

[davep]

Wasat already has the key method: https://wasat.davep.dev/library/certs/#wasat.certs.ClientCertificateStore.register_credentials