[txservice] Support EIP-1559 transactions where possible.

[jakekidd]

Right now we have hardcoded the type of all transactions to 0 (effectively making them legacy). This is to avoid the fact that - if gasPrice is defined and a chain does support EIP-1559 - ethers will set these values like so:

tx.maxFeePerGas = gasPrice; // AKA baseFee
tx.maxPriorityFeePerGas = gasPrice; // maxPriorityFeePerGas

According to sources below, this is what will effectively be paid in gas fees in EIP-1559:

min(gasFeeCap, baseFee + gasTipCap)

It's unclear where the gasFeeCap will come from exactly with ethers' converted transaction. So we need to manage the fee logic depending on whether the chain supports EIP-1559 or not.

Some important links:
ethers-io/ethers.js#1610
https://docs.ethers.io/v5/api/utils/transactions/#Transaction
https://docs.ethers.io/v5/api/providers/provider/#Provider-getGasPrice
https://docs.ethers.io/v5/api/providers/types/#providers-FeeData
https://docs.avax.network/learn/platform-overview/transaction-fees/
https://www.blocknative.com/blog/eip-1559-fees