feat(api): add support for GraphQL filter attributeExists

[5d]

Issue #

This is reopen PR for #3484

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.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 52 lines in your changes missing coverage. Please review.

Project coverage is 68.42%. Comparing base (cb80b91) to head (a6c0a26).
Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
...egories/DataStore/Model/Internal/Persistable.swift	8.33%	33 Missing ⚠️
...ify/Categories/DataStore/Query/QueryOperator.swift	0.00%	12 Missing ⚠️
Amplify/Categories/DataStore/Query/ModelKey.swift	0.00%	2 Missing ⚠️
...mplify/Categories/DataStore/Query/QueryField.swift	0.00%	2 Missing ⚠️
...ries/DataStore/Query/QueryOperator+Equatable.swift	50.00%	1 Missing ⚠️
...fy/Categories/DataStore/Query/QueryPredicate.swift	0.00%	1 Missing ⚠️
...Plugin/Storage/SQLite/SQLStatement+Condition.swift	97.56%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3838      +/-   ##
==========================================
- Coverage   68.44%   68.42%   -0.02%     
==========================================
  Files        1082     1082              
  Lines       37699    37783      +84     
==========================================
+ Hits        25803    25854      +51     
- Misses      11896    11929      +33     

Flag	Coverage Δ
API_plugin_unit_test	69.32% <ø> (ø)
AWSPluginsCore	68.64% <100.00%> (+0.10%)	⬆️
Amplify	47.36% <7.27%> (-0.20%)	⬇️
Analytics_plugin_unit_test	84.52% <ø> (ø)
Auth_plugin_unit_test	79.48% <ø> (-0.02%)	⬇️
DataStore_plugin_unit_test	81.39% <97.77%> (+0.12%)	⬆️
Geo_plugin_unit_test	72.00% <ø> (ø)
Logging_plugin_unit_test	62.95% <ø> (ø)
Predictions_plugin_unit_test	36.86% <ø> (ø)
PushNotifications_plugin_unit_test	86.21% <ø> (ø)
Storage_plugin_unit_test	76.81% <ø> (ø)
unit_tests	68.42% <50.00%> (-0.02%)	⬇️

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