Make GasService Modular

Make GasService modular, with the current implementation (which uses etherscan/polygonscan/node price/chain history RPC to determine gas prices) as a default implementation.

- Any plugin service should be able to override any specific chain(s) method to fetch the gas price, with the default method as a fallback if an error is encountered.

Optionally (but really neat):
- New modules should be submittable by 3rd party and if there are several the user should be able to switch them in the options settings.

As a start try implementing a GasHawk service:

https://www.gashawk.io/

We met with the GasHawk team at Devcon Bogota - they would probably be quite helpful if pinged.