Cronvoy: Negotiated Protocol

As part of the Cronet API, the Cornet Engine exposes the "Negotiated Protocol".

/**
 * Returns the protocol (for example 'quic/1+spdy/3') negotiated with the server.
 * Returns an empty string if no protocol was negotiated, the protocol is
 * not known, or when using plain HTTP or HTTPS.
 */
[public String getNegotiatedProtocol()](https://chromium.googlesource.com/chromium/src/+/refs/heads/main/components/cronet/android/api/src/org/chromium/net/UrlResponseInfo.java#92)

This somehow will need to be resurfaced by the Envoy Mobile Engine. I guess that adding a ```x-envoy-negotiated-protocol``` response header would suffice.