Adds support for struct with both type and fields constraint

[desaikd]

Issue #, if available:

Description of changes:

This PR works on adding support for struct with both type and fields constraint.

List of changes:

• Modified map_constraint_to_abstract_data_type to add support for type: struct.
• Currently implementation returns an error if only type: struct was used without fields constraints.
• Generated code remains same as per fields constraint.

Tests:

Modified current tests and add an invalid test case for only type:struct constraint inside a type def.

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