Support optional External Trigger for counting reference.

[dpwe]

Hello, me again.

To better measure the accuracy of my 10 MHz xtal, I wanted to use an external 1PPS reference signal (from a GPS receiver) to trigger the pulse counts. This change enables this mode, selected by a different initializer call.

I found that the internal interval counter was ~15ppm slow compared to the external 1PPS reference. That's insignificant for many applications, but I'm trying to measure accuracy in the parts per billion.

This change is layered on top of my previous PR to use the ESP32 hardware pulse counter (to allow it to count frequencies as high as 10MHz). However, the change is in principal independent of that change.

[dpwe]

Also, FYI, I ported all this to the RP2040: https://github.com/dpwe/FreqCountRP2

[kapraran]

Thanks for your work 👍