feat(cli): metadata can live in a separate file

[rix0rrr]

When CDK apps grow extremely large (think 10-100 stacks, 1000-10000 constructs), all metadata together begins to exceed 512MB, the maximum string size in NodeJS. People usually deal with this by disabling metadata, but they shouldn't have to.

In addition, even for manifests that don't exceed 512MB the extremely large size of the single JSON object slows down its writing and reading every time, even if the metadata doesn't need to be accessed.

An effective solution is to write the metadata of an artifact to a separate file. This PR introduces the ability for that into the Cloud Assembly schema, and updates the CLI to read from both sources if available.

Removes an unused class from the refactoring project that we found through this project because it would have to be refactored. Rather than refactoring, bye-bye!

Relates to aws/aws-cdk#34480.

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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

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.73%. Comparing base (78fa807) to head (c3494fe).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/aws-cdk/lib/cli/cdk-toolkit.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #568      +/-   ##
==========================================
+ Coverage   87.70%   87.73%   +0.02%     
==========================================
  Files          72       72              
  Lines       10095    10095              
  Branches     1331     1332       +1     
==========================================
+ Hits         8854     8857       +3     
+ Misses       1216     1213       -3     
  Partials       25       25              

Flag	Coverage Δ
suite.unit	87.73% <0.00%> (+0.02%)	⬆️

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