[#283] v2/session: Support ContainerSessionContext message

Define `ContainerSessionContext` structure, implement getters / setters,
JSON and binary encoders, gRPC converters. Support new type of context in
SessionTokenBody message. Add test message generator and cover methods with
unit tests.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

v2/session/json.go

@@ -119,3 +119,11 @@ func (r *ResponseVerificationHeader) UnmarshalJSON(data []byte) error {
 
 	return r.FromGRPCMessage(msg)
 }
+
+func (x *ContainerSessionContext) MarshalJSON() ([]byte, error) {
+	return message.MarshalJSON(x)
+}
+
+func (x *ContainerSessionContext) UnmarshalJSON(data []byte) error {
+	return message.UnmarshalJSON(x, data, new(session.ContainerSessionContext))
+}