feat: added Expo support

[narendracello]

This PR adds support for Expo. One can install cello-react-native using Expo config plugins by adding the below to app.json

{
  "expo": {
    "plugins": ["@getcello/cello-react-native"]
  }
}

The plugin provides props to set environment. Every time you change the props or plugins, you'll need to rebuild (and prebuild) the native app. If no extra properties are added, defaults will be used.

• env (string): Set to your desired environment, such as prod, sandbox. Optional. Defaults to prod.

{
  "expo": {
    "plugins": [
      [
        "@getcello/cello-react-native",
        {
          "env": "sandbox"
        }
      ]
    ]
  }
}

The Readme is also updated accordingly.