Allow passing gasPrice (optional) to getTransactionCost

[danielbate]

In the Fuel Wallet, the user is allowed to pass a custom gas limit at tx approval. Using this new gas limit, it will then calculate the fee. Should this fee be greater than that set initially against the transaction request, it estimates and funds the entire transaction (via getTransactionCost). Source.

Within this flow, the SDK is fetching the gas price twice:

1. Once for fee calculation
2. And again to estimate and fund the transaction

If we allow passing of gasPrice at getTransactionCost we can reduce the redundant gas price call. This is already being done in estimateTxGasAndFee for the same reason.

Relates to #3577

[danielbate]

Closed by #3608