OV2640 standby support

[jksemple]

This issue revisits #101 which was closed due to inactivity.
However, @me-no-dev commented that he 'will implement this very soon' - which was 4 years ago.

This is still a problem and effectively prevents ESP32 Camera being used in any low power / battery role.

[jksemple]

For those struggling with this I have tested OV3660, which does cope with software standby:
sensor->set_reg(sensor, 0x3008, 0x40, enable ? 0x40 : 0x00);

Current drawn by a Xiao ESP32S3 Sense equipped with OV3660 instead of OV2640 drops to less than 5mA in deep sleep after putting the camera into standby mode as above. It is also possible to put the camera into standby mode between pictures which reduces current draw by about 60mA while not in use.

It is likely this code will work with OV5640 too as it has the same registers.

[jksemple]

sensor_t* sensor = esp_camera_sensor_get();
sensor->set_reg(sensor, 0x3008, 0x40, enable ? 0x40 : 0x00);