module-transaction: Add support for initiating PAM Conversations

Modules have the ability to start PAM conversations, so while the
transaction code can handle them we did not have a way to init them.
Yet.

So add some APIs allowing this, making it easier from the go side to
handle the conversations.

In this commit we only support text-based conversations, but code is
designed with the idea of supporting binary cases too.

Added the integration tests using the module that is now able to both
start conversation and handle them using Go only.

cmd/pam-moduler/tests/integration-tester-module/serialization.go

@@ -24,12 +24,30 @@ func (e *SerializablePamError) Error() string {
 	return e.RetStatus.Error()
 }
 
+// SerializableStringConvRequest is a serializable string request.
+type SerializableStringConvRequest struct {
+	Style   pam.Style
+	Request string
+}
+
+// SerializableStringConvResponse is a serializable string response.
+type SerializableStringConvResponse struct {
+	Style    pam.Style
+	Response string
+}
+
 func init() {
 	gob.Register(map[string]string{})
 	gob.Register(Request{})
 	gob.Register(pam.Item(0))
 	gob.Register(pam.Error(0))
+	gob.Register(pam.Style(0))
+	gob.Register([]pam.ConvResponse{})
 	gob.RegisterName("main.SerializablePamError",
 		SerializablePamError{})
+	gob.RegisterName("main.SerializableStringConvRequest",
+		SerializableStringConvRequest{})
+	gob.RegisterName("main.SerializableStringConvResponse",
+		SerializableStringConvResponse{})
 	gob.Register(utils.SerializableError{})
 }