fix: PDU session cannot be established if DNN contains a period(.)

[ianchen0119]

Refer to GitHub Issue #267

• To replace all periods(.) in DNN to underscores(_) before subscription data be sent to the backend.
• To replace all underscores(_) in DNN to periods(.) after subscription data be received from the backend.

After data transmission succeeds, let's check out the sm_data in MongoDB:

we can see that the key of DNN which contains periods(.), was replaced successfully.