feat(api): add support for GraphQL filter attributeExists

[5d]

Issue #

Description

• add support for AppSync GraphQL filter attributeExists
• fix query filter with nil value.
• optimize the SQL query generation for attributeExists and eq(nil)/ne(nil)

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• 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
• 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.