Feature request: BankID proof

BankID is basically the Swedish version of #2126. We use it to authenticate and sign documents with swedish authorities, banks, health centers and all kinds of businesses where identity proof is needed (payment processors, dating apps, gamling, etc).

Linking my keybase identiy with BankID would let me prove that my git commits, emails, tweets etc are actually signed by me which would be pretty neat.

The cost for a BankID login is 0.15 SEK (~0.02 USD).

BankID: https://www.bankid.com/en/om-bankid/detta-ar-bankid
Wikipedia: https://en.wikipedia.org/wiki/Electronic_identification#Sweden
Pricing: https://medium.com/@jessetinell/pris-f%C3%B6r-bankid-6cc3b5264090

Start by reviewing related issue #2126 and the linked BankID background and pricing references. Define how BankID identity proof would link to a Keybase identity and verify commits, emails, tweets, or other signatures; done requires an agreed integration design and scope.

Written by the indexing model from the issue text.