Add DTMF support for QMI modems

Merged Dylan Van Assche requested to merge dylanvanassche/ModemManager:qmi-dtmf into main

QMI modems use native QMI calls for call management, but not yet for DTMF. For this feature, a fallback to AT commands is used, but some modems do not have an AT port.

This MR implements the native QMI calls for DTMF.

Depends on libqmi!327 (merged)

Fixes #543 (closed)

Edited by Dylan Van Assche

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