Wallet Notify integration

[rpavlovs]

These changes add a standardized way of sending push notifications to Metamask using ERC-2365.

Check out walletnotify.org for a demo.

[danfinlay]

Thanks for opening this, it's a good idea to integrate notifications into the wallet, but we aren't quite ready to commit to this strategy.

For one thing, we'd like to preserve a user's right to explicitly consent to receiving such messages, to avoid a spam-promoting environment.

We're also eager to integrate notifications as snaps so that many notification protocols can be experimented with without needing our approval.

Closing for now, because in the current state we aren't ready to merge this.