Parse EXIF metadata for heif_parser

[enricoribelli]

Metadata is already currently retrieved for heif and heic formats.
The set of information returned by the current logic, depending whether such information is contained in the analysed file, is the following (for example):

{
  "nature": "image",
  "format": "heic",
  "width_px": 1440,
  "height_px": 960,
  "display_width_px": 1440,
  "display_height_px": 960,
  "has_multiple_frames": null,
  "orientation": null,
  "has_transparency": null,
  "color_mode": null,
  "num_animation_or_video_frames": null,
  "image_orientation": null,
  "intrinsics": {
    "compatible_brands": [
      "heic",
      "mif1"
    ],
    "handler_type": "pict",
    "pixel_aspect_ratio": null,
    "colour_info": null,
    "pixel_info": [
      {
        "bits_in_channel_2": 8
      },
      {
        "bits_in_channel_3": 8
      },
      {
        "bits_in_channel_4": 8
      }
    ],
    "horizontal_offset": null,
    "vertical_offset": null,
    "clean_aperture": null,
    "rotation": 0
  },
  "content_type": "image/heic"
}

However so far we do not parse the whole EXIF data box, which may contain extra metadata worth showing in the result.