Add Webauthn remoteClientDataJSON extension explainer (#1306)

Adds an explainer for a proposed WebAuthn extension, remoteClientDataJSON, which allows an authorized remote desktop web client to supply the complete clientDataJSON string to the browser so it is passed through verbatim to the platform authenticator. This addresses signature verification failures in remote desktop scenarios, where the browser-constructed clientDataJSON differs from the one the remote host originally passed to the platform authenticator API — a gap that the existing remoteDesktopClientOverride extension does not fully close.

Also updates README.md to list the explainer in the Active Explainers table.

Related spec PR: w3c/webauthn#2375
Related issue: w3c/webauthn#1577

Author: bobomb

.github/ISSUE_TEMPLATE/webauthn-remoteclientdatajson.md

@@ -0,0 +1,10 @@
+---
+name: WebAuthn remoteClientDataJSON Extension
+about: new issue
+title: "[WebAuthn remoteClientDataJSON] <TITLE HERE>"
+labels: WebAuthn RemoteclientDataJSON Extension
+assignees: bobomb
+
+---
+
+

README.md

@@ -61,6 +61,7 @@ we move them into the [Alumni section](#alumni-) below.
 | [CSS ::tooltip Pseudo Element](CSSTooltipPseudo/explainer.md) | <a href="https://github.com/MicrosoftEdge/MSEdgeExplainers/labels/TooltipPseudo">![GitHub issues by-label](https://img.shields.io/github/issues/MicrosoftEdge/MSEdgeExplainers/TooltipPseudo?label=issues)</a> | [New issue...](https://github.com/MicrosoftEdge/MSEdgeExplainers/issues/new?template=tooltip-pseudo.md) | CSS |
 | [CSS Text Transitions & Animations](CSSTextTransitions/explainer.md) | <a href="https://github.com/MicrosoftEdge/MSEdgeExplainers/labels/CSSTextTransitions">![GitHub issues by-label](https://img.shields.io/github/issues/MicrosoftEdge/MSEdgeExplainers/CSSTextTransitions?label=issues)</a> | [New issue...](https://github.com/MicrosoftEdge/MSEdgeExplainers/issues/new?labels=CSSTextTransitions&title=%5BCSSTextTransitions%5D+) | CSS |
 | [Enterprise Platform Authentication Broker API](PlatformAuthentication/explainer.md) | <a href="https://github.com/MicrosoftEdge/MSEdgeExplainers/labels/Enterprise%20Platform%20Authentication">![GitHub issues by-label](https://img.shields.io/github/issues/MicrosoftEdge/MSEdgeExplainers/PlatformAuthentication?label=issues)</a> | [New issue...](https://github.com/MicrosoftEdge/MSEdgeExplainers/issues/new?template=platform-authentication-api.md) | Identity |
+| [WebAuthn `remoteClientDataJSON` Extension](WebAuthnRemoteClientDataJSON/explainer.md) | <a href="https://github.com/MicrosoftEdge/MSEdgeExplainers/labels/WebAuthn%20RemoteclientDataJSON%20Extension">![GitHub issues by-label](https://img.shields.io/github/issues/MicrosoftEdge/MSEdgeExplainers/WebAuthn%20RemoteclientDataJSON%20Extension?label=issues)</a> | [New issue...](https://github.com/MicrosoftEdge/MSEdgeExplainers/issues/new?template=webauthn-remoteclientdatajson.md) | Identity |
 | [Handwriting CSS attribute](Handwriting/HandwritingIntentCSSValue.md) | <a href="https://github.com/MicrosoftEdge/MSEdgeExplainers/labels/Handwriting"> ![GitHub issues by-label](https://img.shields.io/github/issues/MicrosoftEdge/MSEdgeExplainers/Handwriting?label=issues)</a> | [New issue...](https://github.com/MicrosoftEdge/MSEdgeExplainers/issues/new?assignees=adettenb&labels=Handwriting&title=%5BHandwriting%5D+Issue) | CSS |
 | [Pen Events](PenEvents/explainer.md) | <a href="https://github.com/MicrosoftEdge/MSEdgeExplainers/labels/Pen%20Events"> ![GitHub issues by-label](https://img.shields.io/github/issues/MicrosoftEdge/MSEdgeExplainers/Pen%20Events?label=issues)</a> | [New issue...](https://github.com/MicrosoftEdge/MSEdgeExplainers/issues/new?template=pen-events.md) | Editing |
 | [Pen Action](PenAction/explainer.md) | <a href="https://github.com/MicrosoftEdge/MSEdgeExplainers/labels/Pen%20Action"> ![GitHub issues by-label](https://img.shields.io/github/issues/MicrosoftEdge/MSEdgeExplainers/Pen%20Action?label=issues)</a> | [New issue...](https://github.com/MicrosoftEdge/MSEdgeExplainers/issues/new?template=pen-action.md) | Editing |