Add EVMB to Envoy QUIC connection options.

This will allow us to distinguish Envoy Mobile clients from Cronet in the query logs.

PiperOrigin-RevId: 658454097

quiche/quic/core/crypto/crypto_protocol.h

diff --git a/quiche/quic/core/crypto/crypto_protocol.h b/quiche/quic/core/crypto/crypto_protocol.h
index c4971de..6f16950 100644
--- a/quiche/quic/core/crypto/crypto_protocol.h
+++ b/quiche/quic/core/crypto/crypto_protocol.h
@@ -284,6 +284,7 @@
                                                  // preferred address once it is
                                                  // received. Send all coalesced
                                                  // packets to both addresses.
+const QuicTag kEVMB = TAG('E', 'V', 'M', 'B');
 
 // Optional support of truncated Connection IDs.  If sent by a peer, the value
 // is the minimum number of bytes allowed for the connection ID sent to the