Add "open in MetaMask" system extension

Our web3 browser's utility is limited by how often someone is willing to copy and paste a URL into it.

Most browsing is not done in our browser today.

Mobile systems provide ways to add "open in ____" buttons to the share dialog, and this can provide a very user-friendly path to help users open sites in their web3 browser.

I made [an iOS shortcut](https://twitter.com/danfinlay/status/1415815975841132545?s=20&t=EIlNatwZDYqABeLf1Q6A1w) to try this out, and I completely love it, and I think it would be a loved feature to ship in the app, and would probably increase our in-app browser usage.

- [iOS Sharing and Actions extensions](https://developer.apple.com/design/human-interface-guidelines/ios/extensions/sharing-and-actions/)
- [Android sharing extensions](https://support.whisk.com/hc/en-us/articles/360042707631-Enabling-the-Share-Extension-on-Android)

Start by reading the linked iOS Sharing and Actions Extensions and Android sharing extensions guidance, then inspect the mobile app's existing platform integration points. Done means users can choose MetaMask from each platform's share or “open in” flow and the selected URL opens in the app's web3 browser on iOS and Android.

Written by the indexing model from the issue text.