Feat/custom pk flutter impl (#394)

* fix(appsync-dart-visitor): update broken test snapshot

* chore(appsync-dart-visitor): improve indexes generate following Dart lint rules

* feat(appsync-dart-visitor): generate custom PK supporting code

* Make custom PK fields immutable in copyWidth function

* Improve ModelIdentifier methods implementation

* Use the feature flag to control custom PK impl generation

* udpate snapshot for updated dart comment blocks

* Improve generated model identifier helper class

* Update existing unit test snapshots and add new tests for custom PK

* Remove unnecesary new initiating instances

* revert comment block changes

* Update the impl getting identifier fields

* Revert "Remove unnecesary new initiating instances"

This reverts commit 38927cb.

* Remove trash code...