Add OpenAPI spec about awesome-webauthn

[yackermann]

WebAuthn server integration is always nuanced. Everyone doing it their way. For conformance testing FIDO has its own API, but everyone doing it their way.

[Southclaws]

Yes but the spec itself is pretty solid and now that it's an implemented standard, the spec is unlikely to change in a drastic way. So an OpenAPI spec that defines all of the data structures in a conformant way provides value as it allows implementors on either side (client or server) to generate code for their bespoke implementations. An OpenAPI spec is just the data structure schemas, not the logic so it's quite useful to share.