Support adding of only Cognito Identity Pool

Originally opened by @pai911 https://github.com/awslabs/aws-mobile-appsync-sdk-ios/issues/262

**State your question**
Is it possible to use **AWSMobileClient** without using **Cognito User Pool**?

I am migrating from an old Cognito version, so I already set up identity pools for Facebook / Google / Developer Authenticated providers. It works fine with only **Identity Pools**.

In Amplify, **AWSMobileClient** seems to be the correct way to do the login, such as the following code snippet. (https://aws-amplify.github.io/docs/ios/authentication)

`AWSMobileClient.sharedInstance().federatedSignIn`

However, when I tried to use **amplify add auth** to add auth support, I am always asked to provide information for user pool.

Even if I followed the instruction [here](https://aws-amplify.github.io/docs/ios/authentication), I have to provide information for the Cognito User Pool.
```
$ cd ./YOUR_PROJECT_FOLDER
$ amplify add auth ##"amplify update auth" if already configured
❯ Manual Configuration
❯ User Sign-Up, Sign-In, connected with AWS IAM controls
```

**Environment(please complete the following information):**
- AppSync SDK Version: [e.g. 2.6.21]
2.11.0
- Dependency Manager: [e.g. Cocoapods, Carthage]
Cocoapods
- Swift Version : [e.g. 4.0]
4.2

**Device Information (please complete the following information):**
- Device: [e.g. iPhone6, Simulator]
iPhone8 Simulator
- iOS Version: [e.g. iOS 11.4]
iOS 12
- Specific to simulators: