feat(api): add support for GraphQL filter attributeExists (#3484) #562

Job	Run time
GraphQL \| Auth Directive Tests	0s
GraphQL \| User Pool Tests	0s
GraphQL \| IAM Tests	0s
GraphQL \| Lazy Load Tests	0s
REST \| User Pool Tests	0s
Functional Tests / API Integration Tests \| Functional (iOS) / iOS Tests \| AWSAPIPluginFunctionalTests	10m 10s
Functional Tests / API Integration Tests \| Functional (tvOS) / tvOS Tests \| AWSAPIPluginFunctionalTests	9m 36s
Functional Tests / API Integration Tests \| Functional (watchOS) / watchOS Tests \| AWSAPIPluginFunctionalTestsWatch	7m 22s
GraphQL \| Lambda Tests / API Integration Tests \| GraphQL Lambda (iOS) / iOS Tests \| AWSAPIPluginGraphQLLambdaAuthTests	7m 37s
GraphQL \| Lambda Tests / API Integration Tests \| GraphQL Lambda (tvOS) / tvOS Tests \| AWSAPIPluginGraphQLLambdaAuthTests	6m 37s
GraphQL \| Lambda Tests / API Integration Tests \| GraphQL Lambda (watchOS) / watchOS Tests \| AWSAPIPluginGraphQLLambdaAuthTestsWatch	4m 58s
REST \| IAM Tests / API Integration Tests \| REST IAM (iOS) / iOS Tests \| AWSAPIPluginRESTIAMTests	8m 6s
REST \| IAM Tests / API Integration Tests \| REST IAM (tvOS) / tvOS Tests \| AWSAPIPluginRESTIAMTests	8m 53s
REST \| IAM Tests / API Integration Tests \| REST IAM (watchOS) / watchOS Tests \| AWSAPIPluginRESTIAMTestsWatch	4m 59s
	1h 8m 18s

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