Add support for encrypted attachments

[wooorm]

This PR is related to GH-20732, which is about AuthEvent (to delay promting for a password), but instead adds the actual support for encrypted attachments.
“Encrypted attachments” means that the main things are plain text. Note that some PDF viewers, like Preview/QuickLook/Safari or Chrome, do not support attachments at all.
Also note that the file checked into the tests is the same as output-no-auth-event.pdf referenced in
#20139 (comment).

Closes GH-20139.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.19%. Comparing base (bf9ae76) to head (64ecd4c).

Files with missing lines	Patch %	Lines
src/core/parser.js	88.46%	3 Missing ⚠️
src/core/crypto.js	94.11%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21234      +/-   ##
==========================================
+ Coverage   54.70%   56.19%   +1.49%     
==========================================
  Files         216      220       +4     
  Lines       59131    59257     +126     
==========================================
+ Hits        32347    33301     +954     
+ Misses      26784    25956     -828     

Flag	Coverage Δ
fonttest	8.64% <0.00%> (?)
unittest	55.46% <91.66%> (?)
unittestcli	54.74% <91.66%> (+0.03%)	⬆️

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