Update universalWalletConnector to include switchChain method

[robyn3choi]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• Adds the ability to switch chains when using the universal wallet connector
• Exposes the instance of the magic SDK as a property on the connector (it lets you call connector.magic.wallet.showUI() from a React component and have it open the wallet on the newly switched chain)

What is the current behavior? (You can also link to an open issue here)

• Calling switchChain when connected with the universal wallet connector does nothing - Can't switch chain with universalWalletConnector #22.
• Cannot access the magic SDK instance from the connector

**What is the new behavior (if this is a feature change)?

• Calling switchChain when connected with the universal wallet connector switches chains
• Can use the connector to access the most recent instance of magic SDK (so it's always on the correct chain)