[FEATURE] SSTV Implementation

[KDMcMullan]

It would be fantastic to see these little HTs transmit and receive Slow Scan TV pictures.

Given that the firmware supports a webserver, reception of SSTV requires a frequency detector. The ESP32 is not ideal for this, as it's got a lot of interrupt going on in the background, although some claim success with Goertzel algorithm implementations.

The other half of the equation is the potential for SSTV transmission. Again, with a lot going on in the background, the ESP32 based devices are not ideal signal generators, although success has been achieved with the "serial port abuse" principle, as used for rendering audio files on this device.