feat(integ-tests): add integ tests for the toolkit library

[rix0rrr]

Add integration tests for the toolkit library.

Introduces the toolkit library as a new "component under test".

Because we want to promote writing integ tests against the toolkit library the same way we would write normal production code, the setup is like this:

• The toolkit lib is installed as a devDependency into the integ test package. This will allow writing tests as if the toolkit lib is "just" a dependency.
• At runtime, a version of the toolkit library can be selected and it will be installed into the dependency closure by the runner before the tests are run, so that the imports will still function properly.

To signal this, we'll mark @aws-cdk/toolkit-lib as an optionalDependency as well.

(PR includes #444)

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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

[github-actions]

Total lines changed 1113 is greater than 1000. Please consider breaking this PR down.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.18%. Comparing base (7a950ef) to head (0fe8d7e).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #430   +/-   ##
=======================================
  Coverage   79.17%   79.18%           
=======================================
  Files          54       54           
  Lines        6910     6912    +2     
  Branches      772      773    +1     
=======================================
+ Hits         5471     5473    +2     
  Misses       1421     1421           
  Partials       18       18           

Flag	Coverage Δ
suite.unit	79.18% <100.00%> (+<0.01%)	⬆️

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