Flutter wrapper for Metamask SDK(Web, Android, IOS)

# Feature Request

Having a Flutter wrapper package for Metamask SDK(Javascript, Android and IOS) would be really great.

## Why it is needed
Flutter is a great thing to build cross-platform apps on, and a [Web3 library(WebThree](https://github.com/devopsdao/webthree) for dart language is available, also WalletConnect has released its support for Flutter.

## Possible implementation

It would rely on https://github.com/MetaMask/metamask-sdk for Web https://github.com/MetaMask/metamask-android-sdk and https://github.com/MetaMask/metamask-ios-sdk for mobile platforms.

### Code sample

WebThree library already has and [example](https://github.com/devopsdao/webthree/blob/master/example/metamask/web/main.dart) of using Metamask for Web

Review the referenced MetaMask SDK, metamask-android-sdk, metamask-ios-sdk, and the WebThree example in example/metamask/web/main.dart. Compare the platform APIs and determine the wrapper surface needed for Web, Android, and iOS. Done means a Flutter package supports the requested platforms with documented usage and examples.

Written by the indexing model from the issue text.