Allow capturing a JPEG into a bytes object

[brentru]

While capture_into_bitmap() exists, it produces a bmp too large for sending over MQTT . This PR factors out the capture into memory buffer code from capture_jpeg() as capture_into_jpeg(), which returns a bytes object if the capture was successful otherwise None (return type must be checked from user-code).

[brentru]

we might want to make the mode switching lazy someday, or re-use capture_into_jpeg from capture_jpeg, but this is a good start with a handy feature

Yeah, I was considering that