Changes

WIP: TFO frame insert/extract functions

In 2025-02 functions were added for encoding and decoding TFO frames,
which are slightly modified TRAU-UL frames.  Those functions are
slightly modified versions of regular TRAU frame encoding and
decoding functions, implemented in trau_frame layer.  However,
a more useful API for TFO applications (TFO-equipped transcoder
implementations) is one level up: inserting a TFO frame into a block
of 160 G.711 PCM samples or extracting a frame from such sample
block.  This slightly higher level is also more amenable to unit
testing: unit tests can perform decoding of TFO frames captured
from historical E1-based TRAU equipment, providing a cross-check
against actual reality and not just our own reading of the specs.

The present patch adds the desired next-level-up TFO frame insertion
and extraction functions, as well as unit tests that exercise both
the present new addition and the underlying functions added in
2025-02.

The present WIP version is incomplete: TFO frame insertion and
extraction functions are implemented, unit test programs are
implemented for both, but only the decoder unit test set is
properly integrated.  Assistance is sought with integrating
unit tests for the encoder - see tests/tfo/round-trip-test.sh.

Change-Id: Idf149ec682e4064f0e63d67ac757d92402f22dca