adds `new_schema()` for `SchemaSystem` and minor ISL model fixes

[desaikd]

Description of changes:

This PR works on adding new_schema for SchemaSystem and adds minor ISL model related fixes.

List of changes:

• adds new_schema and new_isl_schema in SchemaSystem
• adds fixes for user reserved fields
  • user_reserved_fields should not consider validation for imports.
  • Incorrect error "User reserved field must be non-null and unannotated struct" was thrown when user_reserved_fields didn't even exist.
• adds version information for Annotation
  • this will be useful to determine whether annotation is allowed or not for each annotation in the annotations list
• adds a fix for inline import
  • inline import should not accept as for ISL 2.0
  • inline import should consider as open content for ISL 1.0

Test:

adds tests for new_schema() and new_isl_schema() .

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.