make more configurable

[brad-decker]

This PR does the following:

1. renames 'getIsMainnet' to 'getCurrentNetworkLegacyGasAPICompatibility'
2. adds a 'getChainId' argument to the controller
3. adds support for providing API endpoint URLs int the controller
4. Supports replacing <chain_id> in provided URLs.

Goals:

1. Presumably if the API meets the spec put together in our mock data, we can swap out URLs without rebuilding/re-releasing
2. Support networks like Binance that has its own gas estimation API.

[wachunei]

LGTM