Simplify Free Gas Network Configuration

[siladu]

This PR was recently merged #4003

However the end result is a clunky UX for configuring gas free networks. It would be good to get this right before updating the documentation: https://besu.hyperledger.org/en/stable/HowTo/Configure/FreeGas/#3-start-besu-with-a-minimum-gas-price-of-zero

It would be good to make this simpler and more explicit, e.g. a single config parameter that replaces these two:
--min-gas-price=0 --override-genesis-config="baseFeePerGas=0x0"

For example --zero-price-gas or --free-gas.

One approach would be to have this new parameter simply set the other two under the hood.

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UPDATE: the approach below is being looked at as a spike, #4067, so could do the simpler approach above as a quick win that could then be documented. The new cli flag could be reused for the approach below.

Another would be to replace LondonFeeMarket with a ZeroFeeMarket (or just wire up LegacyFeeMarket instead?)

Switching out the fee market seems cleaner and may solve this network upgrade issue, but it may be a lot more error prone than just zeroing the baseFee, since certain "London behaviour" may assume the baseFee is present.

[siladu]

Config changes will now be part of #4061