Support alternate gas price oracle implementations

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Currently, the node determines the gas price to use for transactions by asking its ETH JSON-RPC provider for a gas price suggestion using the eth_gasPrice RPC call. The gas price returned by this RPC call may not be a very good estimate of the gas price required to get a transaction in a block since the estimation for this RPC call will depend on the data that the ETH JSON-RPC provider has available. As a result, the node may end up overpaying for transactions by using a gas price that is higher than what is actually required.

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A few options to consider for improving on the status quo:

• Support integrating with services such as https://www.gasnow.org/, https://ethgasstation.info/ or https://etherscan.io/gastracker
• Add support for integrating your own custom gas price oracle. This could be useful if you're running your own geth node since you can configure the gas price oracle settings on geth with CLI flags

Start by locating the node's current use of the eth_gasPrice JSON-RPC call. Compare the requested service integrations with the custom oracle option and clarify the configuration and selection behavior before implementation. Done means the node can use an alternate gas price oracle instead of relying only on the provider suggestion.