webauthn signatures: SecurityKeyProvider, json parsing

[Damien Miller]

On Tue, 11 Jan 2022, Peter Stuge wrote:

> FWIW I think the data spray and the complexity are mad, each on their own.
> 
> I guess that it'll be popular, I hope not in mainline OpenSSH. ;)

The SecurityKeyProvider interface exists largely to make this sort of
experimentation possible outside of OpenSSH itself.

-d