Support for integration of state issued certificates

In some European countries (to my knowledge, maybe also elsewhere) governments are issuing certificates which can serve as a way to sign documents with digital signatures which are legally equivalent to normal signatures.

The issue is that there is little support for tooling around such certificates (which are normal public/private keypair, signed by state CA). So it would be great if one could integrate them somehow into keybase ecosystem so that you could:
- prove your keybase identity by signing it with the state-issues certificate
- could alternatively use state-issued key pair for your keybase signatures (so that you could use all common keybase tools, just use state-issued key pair to sign documents, for example)

The issue names no files, tests, or entry points. Start by reviewing the requested identity-proof and signing use cases, then establish the project area and acceptance criteria with maintainers. Done should be defined for the agreed scope, since the issue currently proposes two broad integration options.

Written by the indexing model from the issue text.