Adds a new config option `--l1-gas-price-backend` or
`DATA_TRANSPORT_LAYER_L1_GAS_PRICE_BACKEND` that can be set to `l1` or
`l2`. This impacts the behavior of the HTTP endpoint `GET /eth/gasprice`
by changing what is queried to return the L1 gas price. The L1 gas price
is required to compute the L2 fee since the L2 fee consists of
`L1 gas price * L1 gas used + L1 gas price * L2 gas limit`. If the L1
gas price differs too much between different L2 providers, then users
using `eth_estimateGas` may submit transactions with too low of a fee
and be unable to submit transactions to the sequencer.

By configuring the DTL to use L2 as the L1 gas price backend, it will
call the Sequencer's RPC endpoint `rollup_gasPrices` which returns the
L1 and L2 gas prices from the point of view of the sequencer. The L2 gas
price exists in the state, so that will always be the same between the
sequencer and any replicas. The L1 gas price does not live on chain, so
querying for it from the sequencer directly will ensure that users send
transactions with a fee that is large enough.

Also adds eth/gasprice info to README.