Introduce version extension (#236)

## Release notes: usage and product changes
We introduce the "extension" field into the protocol. This introduces a finer notion of "compatibility" and makes the protocol aware of it.
A driver-server pair is compatible if they are on the same protocol version, and the server extension version is atleast that of the client.

Author: krishnangovindraj

proto/connection.proto

@@ -15,6 +15,7 @@ message Connection {
   message Open {
     message Req {
       Version version = 1;
+      ExtensionVersion extension_version = 5;
       string driver_lang = 2;
       string driver_version = 3;
 

proto/version.proto

@@ -11,3 +11,8 @@ enum Version {
   UNSPECIFIED = 0;
   VERSION = 7;
 }
+
+enum ExtensionVersion {
+  UNSPECIFIED_EXTENSION = 0;
+  EXTENSION = 1; // Reserving makes little sense here.
+}