fix: added @default schema validations

[bzsurbhi]

Description of changes

Adde schema validations for

• @default directive

  • The @default directive may only be added to object definitions annotated with @model
  • The @default directive may only be added to scalar or enum field types
  • The @default directive only takes a value property
  • String cannot represent a non string value: the @default directive has a non String value

• ownerField should be of type String

CDK / CloudFormation Parameters Changed

N/A

Description of how you validated changes

Checklist

• [x] PR description included
• [x] yarn test passes
• [x] Tests are changed or added

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