protobuf: add opts.proto

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

protobuf/opts.proto

+import "google/protobuf/descriptor.proto";
+
+message CRIU_Opts {
+	optional bool hex = 1; // Idicate that CRIT should treat this field as hex.
+}
+
+extend google.protobuf.FieldOptions {
+	// Registered unique number to use for all kinds of custom options.
+	optional CRIU_Opts criu = 1018;
+}