Non-modal "conditionally mediated" UI (#1576)

Add conditional UI flow

This PR enables a non-modal "conditionally mediated" UI feature for WebAuthn which RPs may utilize to provide a credential selection UI only if the user has a discoverable credential registered with the Relying Party on their authenticator (the latter being the "condition"). The credential is displayed alongside an autofilled username or password input field. This helps RPs solve the "bootstrapping problem" when migrating their user base from traditional username and password to WebAuthn: websites can fire a WebAuthn call while showing their typical username and/or password prompt without worrying about showing a modal dialog error if the device lacks appropriate credentials.

See also:
https://github.com/w3c/webauthn/wiki/Explainer:-WebAuthn-Conditional-UI
Fixes #1545

Overview "omnibus" issue: #1637
See also discussion in Issues #1356 #1533 #1568

Co-authored-by: Nina Satragno <nsatragno@gmail.com>
Co-authored-by: Jeff Hodges <nsatragno@gmail.com>
Co-authored-by: Emil Lundberg <emil@emlun.se>
Co-authored-by: Matthew Miller <mmiller@duosecurity.com>