Improve Handling of `ethCompatible`

[trianglesphere]

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Description

Split out LegacyTx into CeloLegacyTx and EthCompatibleLegacyTx. This is done at the golang level. Then at the JSON RPC API level work should be done to clean up the ethCompatible field and how it's created and assigned.

Context

In the Donut HF, Celo add the ability to receive native Ethereum transactions. In the berlin HF, ethereum added typed transactions, further complicated the way of how to take a half filled in transaction and turn it into a canonical encoding.

This area of focus is the JSON RPC API and modifying the golang representation to accurately fit it.