Supporting Klaytn network

[hwnahm]

According to Klaytn network manual (https://docs.klaytn.com/bapp/json-rpc/api-references/klay/transaction)
they returns the transaction type value as a string.

Because of this when get receipt this error happens:
Unhandled Rejection (Error): invalid BigNumber string (argument="value", value="TxTypeLegacyTransaction", code=INVALID_ARGUMENT, version=bignumber/5.5.0)

I think just adding a line of code which is not perfect but mitigates the error:

At 195 line of providers/src.ts/formatter.ts
if (typeof(number) === "string") { return 0; }

and 126 line need to be:
cumulativeGasUsed: Formatter.allowNull(bigNumber);

[benjioh5]

On #644 , there was a some discussion. And Klaybank made some fork of ethersjs for klaytn.

https://github.com/klaybank/ethers_providers_klaytn

[ricmoo]

FYI: This is coming in v6, which is far more flexible in its network support. :)