fix(datastore): add required fields in selection set for hasOne-belongsTo relationship

[thisisabhash]

Issue #

https://github.com/aws-amplify/amplify-swift/issues/3259

Description

This PR adds the required fields(recursively) for an associated model connected to a model with @belongsTo and @hasOne directive.

General Checklist

• [ ] Added new tests to cover change, if needed
• [x] Build succeeds with all target using Swift Package Manager
• [x] All unit tests pass
• [ ] All integration tests pass
• [x] Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• [ ] Documentation update for the change if required
• [x] PR title conforms to conventional commit style
• [ ] New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• [ ] If breaking change, documentation/changelog update with migration instructions

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