Implement the Digital Credentials API (navigator.credentials with DigitalCredential type) for in-browser mDL / VC requests

Steps

1. The Digital Credentials API (formerly digital identity API) extends the Credential Management API to allow websites to request verifiable credentials (mDL, VCs) from a user's wallet app via the browser as intermediary.
2. Call navigator.credentials.get({ digital: { requests: [{ protocol: 'openid4vp', data: <OID4VP_request_JSON> }] } }) from within a user gesture handler; the browser presents a system picker of available wallets.
3. The data field contains an OID4VP Authorization Request object (without redirect_uri — the browser handles the response routing); include presentation_definition, nonce, and client_id.
4. The browser invokes the selected wallet app, which processes the request and returns the vp_token to the browser, which returns it as the result of the navigator.credentials.get call.
5. Verify the returned vp_token server-side: validate the OID4VP response including nonce binding, credential format, issuer signature, and holder binding proof.