Add example for RPC call to get current market info

[kevjin]

This way extensions can know what market the main app is currently on, and handle that for the extension properly (e.g. whether the market is currently set to "ETH" or "USDC")

We need to have a sync call for (requested) 'pull' messages via rpc.sendRPC and handle that resolved data.

• This is necessary to make sure we send data after the extension app is loaded.
  We also need to have an async listener for (unprompted) 'push' messages, via rpc.on(...).
• This is necessary in case the data from the main app gets updated, or if the user made an RPC request and the data wasn't ready yet (e.g. market data hasn't been loaded), but is ready at a later time.