Accept optional dir of coverage test inputs

[ranweiler]

Add a new --input-dir/-d argument to the coverage recording example exe. When provided, the exe will record the combined coverage of every file found in input_dir, substituting the file path for the special marker @@. When not provided, the target command will be executed exactly as-is (the previous behavior).

[codecov-commenter]

Codecov Report

Merging #2853 (35ab044) into main (572671b) will increase coverage by 1.48%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2853      +/-   ##
==========================================
+ Coverage   26.59%   28.08%   +1.48%     
==========================================
  Files         125      302     +177     
  Lines       13414    35653   +22239     
==========================================
+ Hits         3568    10012    +6444     
- Misses       9846    25641   +15795     

Impacted Files	Coverage Δ
src/agent/onefuzz/src/syncdir.rs	10.90% <0.00%> (ø)
...c/agent/onefuzz-file-format/src/coverage/source.rs	66.66% <0.00%> (ø)
...nt/onefuzz-task/src/tasks/report/dotnet/generic.rs	0.00% <0.00%> (ø)
src/agent/onefuzz-agent/src/worker.rs	57.94% <0.00%> (ø)
src/agent/onefuzz/src/heartbeat.rs	0.00% <0.00%> (ø)
src/agent/win-util/src/wer.rs	0.00% <0.00%> (ø)
src/agent/onefuzz/src/machine_id.rs	20.51% <0.00%> (ø)
src/agent/onefuzz-task/src/tasks/merge/generic.rs	0.00% <0.00%> (ø)
...ent/onefuzz-task/src/local/libfuzzer_regression.rs	0.00% <0.00%> (ø)
...nt/onefuzz-task/src/tasks/merge/libfuzzer_merge.rs	0.00% <0.00%> (ø)
... and 167 more

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