feat: impl setGetAppCallback

[killagu]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

[codecov]

Codecov Report

Base: 82.88% // Head: 82.97% // Increases project coverage by +0.09% 🎉

Coverage data is based on head (cae8280) compared to base (9a5fcca).
Patch coverage: 89.02% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
+ Coverage   82.88%   82.97%   +0.09%     
==========================================
  Files          27       27              
  Lines        2465     2531      +66     
  Branches      409      428      +19     
==========================================
+ Hits         2043     2100      +57     
- Misses        422      431       +9     

Impacted Files	Coverage Δ
lib/inject_context.js	88.11% <83.63%> (-5.76%)	⬇️
app/middleware/cluster_app_mock.js	91.46% <100.00%> (ø)
bootstrap.js	100.00% <100.00%> (ø)
index.js	96.80% <100.00%> (+0.06%)	⬆️
lib/app.js	96.92% <100.00%> (+0.01%)	⬆️
lib/app_handler.js	69.09% <100.00%> (+9.56%)	⬆️
lib/mock_agent.js	100.00% <100.00%> (ø)
lib/request_call_function.js	62.50% <100.00%> (-1.50%)	⬇️
register.js	100.00% <100.00%> (ø)

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[github-actions]

🎉 This PR is included in version 5.10.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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