add manual SDK initialization

[jasonpraful]

Changes

• Adds support for manually initialising Intercom SDK.
• Updates expo example to use manual initialization

Example

import { Platform } from 'react-native';
import Intercom from '@intercom/intercom-react-native';

const apiKey = Platform.select({
  ios: 'ios_sdk-YOUR_IOS_KEY',
  android: 'android_sdk-YOUR_ANDROID_KEY',
});

await Intercom.initialize(apiKey, 'YOUR_APP_ID');

For Expo Consumers:

{
  "expo": {
    "plugins": [
      ["@intercom/intercom-react-native", {
        "useManualInit": true
      }]
    ]
  }
}

This PR builds upon the foundation in PR #305.

[lnaia]

just nit, LGTM