GC0308: add explicit constructor to enable custom camera configs

[mhaberler]

the way this was coded made the camera_config_t* config; member variable useless, as it is never set or used.

with this change:

• the constructor can optionally accept a pointer to a custom camera_config_t struct
• when using the default camera_config, it can be accessed and changed via the config member variable or by passing a custom config via begin
• the used config struct can always be accessed via the config member.

Here is a demo for the CoreS3 which reads and decodes a QRcode via the builtin camera, and connects to WiFi if the qrcode was a Wifi URI:

https://github.com/mhaberler/cores3-read-qrcode.git

this uses the patch attached to customize the camera config (resolution, grayscale):

[mhaberler]

ping