Initial work to implement Sapphire snap connection

[CedarMist]

fixes #389

This provides the decryption keys to snap.

Warning

If an RPC server pretends to implement the MetaMask snap protocol it could trick users into revealing the transaction encryption key.

For this reason, we have to explicitly enable Snap support in the dApp, by passing the enableSapphireSnap option.

Usage:

wrapEthereumProvider(window.ethereum, {enableSapphireSnap:true})

This must only be done if the dApp is sure that the provider it's connecting to is MetaMask.

TODO:

• Automated tests for wallet_invokeSnap
• Use flask for testing https://docs.metamask.io/snaps/get-started/install-flask/

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