report: Collect open file descriptors in crash reports

[basil]

systemd-coredump(8) provides information about open file descriptors via COREDUMP_OPEN_FDS in the following format:

fd:/path/to/file
pos: ...
flags: ...
...

fd:/path/to/file
pos: ...
flags: ...
...

The first line contains the file descriptor number fd and the path, while subsequent lines show the contents of /proc/pid/fdinfo/fd. This can be very useful during postmortem debugging.

This PR implements the same feature for Apport. We read /proc/pid/fd and /proc/pid/fdinfo to populate a new OpenFds field with each file descriptor's target path and metadata. We also map COREDUMP_OPEN_FDS from systemd-coredump(8) into the same field.

Unit and integration tests have been updated to cover the new functionality, as well as the specification in doc/data-format.tex.

Fixes 2143109

CC @bdrung

[basil]

CLA checks, linter, and ruff are passing, as are unit and integration tests on all platforms. System tests timed out trying to reach https://api.launchpad.net/devel/ubuntu/ but this is a transient failure not caused by my changes.

[codecov]

Codecov Report

❌ Patch coverage is 88.67925% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.41%. Comparing base (61ee6d6) to head (dd61751).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
apport/report.py	84.21%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #555      +/-   ##
==========================================
+ Coverage   84.40%   84.41%   +0.01%     
==========================================
  Files         104      104              
  Lines       20848    20901      +53     
  Branches     3259     3265       +6     
==========================================
+ Hits        17597    17644      +47     
- Misses       2813     2818       +5     
- Partials      438      439       +1     

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[bdrung]

Merged (added bug reference and signed commit)