Pancakeswap: use DEX on-chain data

[bryanjowers]

We recently built an EA for Curve DEX (as the data source) to read price pair data on-chain.

This ticket: We should use the Curve EA as an example and create a new EA for Pancakeswap as the data source. This will be the 3rd DEX after Curve and #823 Uniswap.

Part of this ticket will be investigating Pancakeswap to design the implementation as it might be different than Curve's or require additional work. Specifically, Pancakeswap is on Binance

How we handled Curve DEX:
Curve has a single address provider, which gives us an exchange rate registry contract.
We can provide an input token address, an output token address, along with an amount (depth). It returns the pool used along with the price at that depth.

To note, similar to Amberdata, this requires a token address mapping, since the contract accepts addresses, not token names. Alternatively, you can provide the EA with the token address directly in the request.

@boxhock built Curve's new EA and can assist in answering questions around implementation, and for CR.