WebAuthn signature support

Instructions

The Web Authentication API (also known as WebAuthn) is a specification written by the W3C that allows servers to register and authenticate users using public-key cryptography instead of a password.
It is already supported by all major browsers.

The attractive feature of WebAuthn is being able to use a hardware signing device in a browser without browser extensions or other installed software to sign data.

Issue To Be Solved

Update Flow to verify signatures created by a WebAuthn assertion.
https://developer.mozilla.org/en-US/docs/Web/API/Web_Authentication_API/Attestation_and_Assertion

(Optional): Suggest A Solution

The WebAuthn creates a signature over the data from the client and the data created/used by the authenticator.
Add a way to send extra data to Flow when verifying signatures. The extra data field would be used to send the data created/used by the authenticator.

(Optional): Context

Allow people to sign transactions in browsers, without any extensions.

No files, tests, or entry points are identified. Start by locating Flow's existing signature-verification path and compare it with the WebAuthn assertion reference; define how authenticator data is supplied and verify completion with coverage for WebAuthn assertions.