Nested server structure member shapes targeting simple shapes with default trait (#2352)

* Nested server structure member shapes targeting simple shapes with `@default`

Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>

* Add changelog entry

Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>

* Update CHANGELOG

Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>

* Add unit test

Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>

* Add integration test

Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>

* Change method

Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>

* Include comment to describe the test

Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>

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Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>