Update pdfium to 6392, implement new methods

[jerbob92]

This updates pdfium to version 6392 and implements the following new experimental methods:

• FPDFAnnot_AddFileAttachment
• FPDFAnnot_GetFileAttachment

[codecov]

Codecov Report

Attention: Patch coverage is 79.22078% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 78.14%. Comparing base (fc9a455) to head (5c9cdfa).

Files	Patch %	Lines
internal/implementation_webassembly/fpdf_annot.go	76.19%	5 Missing and 5 partials ⚠️
internal/implementation_cgo/fpdf_annot.go	82.85%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
+ Coverage   78.13%   78.14%   +0.01%     
==========================================
  Files         110      110              
  Lines       18865    18942      +77     
==========================================
+ Hits        14740    14803      +63     
- Misses       2352     2358       +6     
- Partials     1773     1781       +8     

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