Adding extra container to tasks

[chkeita]

Summary of the Pull Request

• Adds an extra readonly container to all tasks
• The container will be synced by the agent, same as the setup_dir
• Adds a new variable substitution to reference it
• Updates the documentation to add the new {extra} variable substitution
• Updated some of the integration test to reference the extra container. The content is ignored but it allows us to test the variable substitution.

closes #2827

[codecov-commenter]

Codecov Report

Merging #2847 (a271dcc) into main (6cd53b1) will decrease coverage by 0.10%.
The diff coverage is 14.70%.

@@            Coverage Diff             @@
##             main    #2847      +/-   ##
==========================================
- Coverage   28.13%   28.04%   -0.10%     
==========================================
  Files         302      302              
  Lines       35577    35801     +224     
==========================================
+ Hits        10011    10041      +30     
- Misses      25566    25760     +194     

Impacted Files	Coverage Δ
src/ApiService/ApiService/OneFuzzTypes/Enums.cs	32.39% <ø> (ø)
src/ApiService/ApiService/OneFuzzTypes/Model.cs	69.44% <0.00%> (-0.18%)	⬇️
src/ApiService/ApiService/onefuzzlib/Config.cs	0.00% <0.00%> (ø)
src/ApiService/ApiService/onefuzzlib/Defs.cs	0.00% <0.00%> (ø)
src/ApiService/ApiService/onefuzzlib/Scheduler.cs	15.16% <0.00%> (-0.27%)	⬇️
src/agent/coverage/src/binary.rs	47.19% <0.00%> (ø)
src/agent/onefuzz-agent/src/debug.rs	0.00% <0.00%> (ø)
src/agent/onefuzz-agent/src/setup.rs	0.00% <0.00%> (ø)
src/agent/onefuzz-task/src/local/common.rs	0.00% <0.00%> (ø)
...ent/onefuzz-task/src/local/libfuzzer_test_input.rs	0.00% <0.00%> (ø)
... and 29 more

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