Add a getGasConfig helper on the Provider class

[Dhaiwat10]

Follow-up from #1181 and this comment:

Possible implementation:

getGasConfig() {
  // get cached info
  const chain = this.getChain();
  const nodeInfo = this.getNodeInfo();

  const { minGasPrice } = nodeInfo;
  const { maxGasPerTx, maxGasPerPredicate, gasPriceFactor, gasPerByte } = chain;

  // return relevant gas info
  return {
    minGasPrice, maxGasPerTx, maxGasPerPredicate, gasPriceFactor, gasPerByte
  }
}

Possible usage:

const provider = await Provider.create(url);
const gasConfig = provider.getGasConfig();

// now use correct config to create things with correct default gasConfig
const scriptRequest = new ScriptTransactionRequest({ gasPrice: gasConfig.minGasPrice, gasLimit: maxGasPerTx });