(MINOR) allow clients to sign with a process outside of c2patool

[dyro]

Changes in this pull request

This PR introduces a way for clients to sign claim bytes using an external process. There are two open questions I have:

1. I had to introduce a couple of bins for integration tests. The problem is one of these binaries, namely the signer-process-success uses openssl, so I had to add that as a dependency to c2patool. I tried adding these as examples, but cargo examples aren't compiled before running cargo test. Those tests would fail unless we ran cargo build --examples; cargo test.
2. I don't think we can calculate the --reserve-size argument dynamically. This will change if a user includes a custom TSA URL. In claims-signer we hard-coded a value, but I'm worried about doing that here. Any suggestions? I'm currently requiring the user to supply the value, is that okay? We could also use the cert size for the reserve size if there's no TSA url in the sign config and only require the reserve size argument if the user specified a tsa url.

I'll write docs after we resolve the reserve-size question!

Checklist

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

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Codecov Report

Attention: Patch coverage is 62.73885% with 117 lines in your changes missing coverage. Please review.

Project coverage is 46.31%. Comparing base (effdb8d) to head (fd8e890).
Report is 46 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bin/signer-path-success.rs	0.00%	52 Missing ⚠️
src/callback_signer.rs	80.08%	49 Missing ⚠️
src/main.rs	0.00%	11 Missing ⚠️
src/bin/signer-path-fail.rs	0.00%	3 Missing ⚠️
src/bin/signer-path-no-stdout.rs	0.00%	2 Missing ⚠️

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HEAD has 6 uploads less than BASE

Flag	BASE (effdb8d)	HEAD (fd8e890)
	9	3

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #169       +/-   ##
===========================================
- Coverage   76.10%   46.31%   -29.80%     
===========================================
  Files           3        7        +4     
  Lines         406      719      +313     
===========================================
+ Hits          309      333       +24     
- Misses         97      386      +289     

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