chore(TestModels): Add constraints inside list, map and union

[rishav-karanjit]

Issue #, if available:

Description of changes:
This PR adds constraints inside list (ListWithConstraint), map (MapWithConstraint) and union (UnionWithConstraint) in the test model.

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[rishav-karanjit]

Self commenting a note posterity:
MyString has constraint @length(min: 1, max: 10)

Other aggregate shapes that was added also has similar constraint

[alex-chew]

It would be good to also add a constraint within multiple levels of structures, to ensure that we're not only validating input structures. For example

structure GetConstraintsInput {
  ... // existing members omitted for brevity
  inner: InnerStructure,
}

structure InnerStructure {
  str: NonEmptyString,
}