fix(auth): rename conflicting name AWSAuthService to AWSCognitoAuthService

[lawmicha]

Issue #, if available:

Description of changes:
This is impacting some build systems when consuming AWSPluginsCore and AWSCognitoAuthPlugin
error:

Multiple commands produce 'AWSAuthService.o':
1) Target 'AWSPluginsCore' (project 'Pods') has compile command for Swift source files
2) Target 'AmplifyPlugins' (project 'Pods') has compile command for Swift source files

renamed file to the class in the file:

public enum AWSCognitoAuthService {

    case awsMobileClient(AWSMobileClient)

}

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