JXR: added support to EXIF metadata

Improved metadata support via EXIF ​​metadata. Since JXR is based on a TIFF container, EXIF ​​data is read directly from the file so it always works (even with versions of libjxr that don't have the metadata reading API).

It also solves the following issues:
- Incorrect date format on saved JXR files (was saved in ISO format instead of `yyyy:MM:dd HH:mm:ss`).
- Incorrect date type setting in EXIF ​​data: the `DateTime` tag should be updated on every save (verified by GIMP and Photoshop). Our `CreationDate` metadata is the equivalent of the EXIF ​​`DateTimeOriginal` tag.

Closes #22

autotests/read/avif/metadata.avif.json

@@ -3,7 +3,7 @@
         "fileName" : "metadata.png",
         "metadata" : [
             {
-                "key" : "CreationDate",
+                "key" : "ModificationDate",
                 "value" : "2025-02-19T08:27:22+01:00"
             },
             {