Support for evm tx custom fields

[sirpy]

Describe the Feature

Some blockchains can support extra fields in the tx request like on Celo "feeCurrency".
In #1761 this was partially addressed but it was assumed the evm would parse the 'customData' field.
It makes more sense that customData would be flattened as part of the final tx before being encoded.

Code Example

sendTransaction({to:...,feeCurrency:'0x....333'})

[arthurgousset]

This issue is probably a duplicate of this initiative:

• Support for EIP-2718 transaction envelope types beyond 1 and 2 #3747

If that issue is closed, your specific need should be satisfied @sirpy.
Do chime in on the other issue, we'd love to hear your use case and opinion :)

[sirpy]

@arthurgousset thanks closing as dup of #3747