NethVoice: Add support for Grandstream HT841TLS (FXO)

[andrea-marchionni]

Description

Add support for the Grandstream HT841TLS analog telephone adapter in NethVoice.

Steps to reproduce

1. Connect a Grandstream HT841TLS device to the network.
2. Try to provision/configure it through NethVoice.
3. Verify that the device is not currently supported.

Expected behavior

NethVoice should recognize and provision the Grandstream HT841TLS.

Actual behavior

The Grandstream HT841TLS is not supported.