Integrate ElevenLabs trunk support into NethVoice

[andrea-marchionni]

We would like to improve the ElevenLabs integration in NethVoice with the following enhancements:

1. Add ElevenLabs trunk to configurable trunks via UI
   Currently, the ElevenLabs trunk is not available among the trunks that can be configured through the NethVoice web interface.
   Please add the ElevenLabs trunk to the list of trunks that can be managed from the UI, so it can be configured without manual intervention.

2. Update transfer mode to also check the Trunk context when resolving the transfer context, since an extension using a modified dial string (e.g., PJSIP/TRUNK_NAME) can initiate a transfer.

3. Documentation: internal extension dialing directly via trunk
   Add documentation explaining how to create an internal extension that dials directly through the ElevenLabs trunk.