ethclient: add GetProof method

[p4u]

Exposes EIP 1186 eth_getProof method to ethclient ethereum/EIPs#1186

[MariusVanDerWijden]

Rel: #21738
Imo instead of adding this to the ethclient we should create a seperate interface for all geth specific methods

[p4u]

Yeah, that makes sense. However it would require a refactor and a big effort. I'd suggest to incorporate mean-while this implementation or else we could return a byte array from json.Marshal() in order to keep the caller away from the internal ethapi.AccountResult type

[fjl]

I don't want to add a temp workaround. The refactoring is not that large, we just need someone to get this gethclient package started and add the one method they want. @fxfactorial already offered to do it in #21738 in order to get a wrapper for the eth_call with overrides. Once that's in, we can add GetProof as well.

[p4u]

fine, thanks.