feat(cli): concurrent asset builds

[go-to-k]

Fixes #226

This PR adds the --asset-build-concurrency option to cdk deploy to parallelize asset build.

cdk deploy --asset-build-concurrency 4

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.78%. Comparing base (c8f40dc) to head (dccb3d1).
⚠️ Report is 79 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #983      +/-   ##
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+ Coverage   87.75%   87.78%   +0.02%     
==========================================
  Files          72       72              
  Lines       10136    10148      +12     
  Branches     1339     1340       +1     
==========================================
+ Hits         8895     8908      +13     
  Misses       1216     1216              
+ Partials       25       24       -1     

Flag	Coverage Δ
suite.unit	87.78% <100.00%> (+0.02%)	⬆️

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[xrn]

@go-to-k one more time thank you for this PR but --asset-build-concurrency does not apply to ILocalBundling that can be missleading

TryBundle is called synchronously during synthesis inside the app process, before the CLI's work graph executes. Assets built locally via ILocalBundling are already resolved by the time --asset-build-concurrency takes effect. This means the flag only helps Docker-based bundling. Go/Rust CDK apps that use ILocalBundling (the recommended approach to avoid Docker overhead) cannot benefit from parallel asset builds. Could the framework defer local bundling to the work graph as well?

[go-to-k]

@xrn
You're right that --asset-build-concurrency only applies to Docker-based asset builds in the CLI's work graph, not to ILocalBundling.

Deferring local bundling to the work graph is a very appealing idea. However, ILocalBundling.tryBundle() is called synchronously during synthesis inside the CDK app process, which is a completely separate phase from the CLI's work graph execution. This would require changes on both the framework side (aws-cdk-lib) and the CLI side, which is a significant architectural change beyond the scope of this PR.

It might be worth opening a separate feature request issue for that so the maintainers can weigh in on the approach. In the meantime, a note in the documentation clarifying that this flag applies to Docker-based builds only could also help avoid confusion.