Add support for PNG metadata

[allgood]

Project now supports generating lossless PNG images, but this format isn't supported by ExifInterface methods.

Recently the folks that regulates the PNG format issued an standard for Exif tags on it, but I was unable to find any Android implementation for it, if somebody knows how to, help is wanted.

[ghost]

As I recall the program writes a "created with OpenNoteScanner" comment to tags, but I don't see custom fields in a JPG file I just created:

$ mediainfo ~/scans/DOC-20221014-162606.jpg 
General
Complete name                            : ~/scans/DOC-20221014-162606.jpg
Format                                   : JPEG
File size                                : 1.48 MiB

Image
Format                                   : JPEG
Width                                    : 2 592 pixels
Height                                   : 4 608 pixels
Color space                              : Y
Bit depth                                : 8 bits
Compression mode                         : Lossy
Stream size                              : 1.48 MiB (100%)

Could you clarify what is added by default?

Hopefully there will be setting to disable any metadata, if not possible already.