[RFC] Enable usage of externally supplied 'TONES DATA'

[kholia]

[RFC] Enable usable of externally supplied 'TONES DATA'

Motivation: Currently, ft8_lib cannot encode free-text messages.

Usage:

$ ft8code "U09TUNQUC9ZH"
...
3140652 00545736465273610247324500636 3140652 55505324500472027752070701676 3140652

$ export DEBUG_TONES_DATA="3140652 00545736465273610247324500636 3140652 55505324500472027752070701676 3140652"

$ ./gen_ft8 "U09TUNQUC9ZH" compatible.wav
Note: DEBUG_TONES_DATA is set! Using it to generate the WAVE file...
FSK tones: 3140652005457364652736102473245006363140652555053245004720277520707016763140652

WSJT-X then decodes the generated compatible.wav file just fine.