Add `--batch` flag for batch signing

[ok-nick]

Changes in this pull request

Adds --batch flag, which can be passed with an input and output directory to sign all children assets of a directory with the same manifest.

Checklist

• [x] This PR represents a single feature, fix, or change.
• [x] All applicable changes have been documented.
• [x] Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[mauricefisher64]

Looks good. Please update the README to include the new command.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 123 lines in your changes missing coverage. Please review.

Project coverage is 46.03%. Comparing base (f216d3e) to head (57f48fc).

Files	Patch %	Lines
src/main.rs	0.00%	123 Missing :warning:

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #183      +/-   ##
==========================================
- Coverage   47.89%   46.03%   -1.87%     
==========================================
  Files           4        4              
  Lines         666      693      +27     
==========================================
  Hits          319      319              
- Misses        347      374      +27     

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[ok-nick]

Closed for now in favor of #185