The fees are currently calculated as a sum of the L1 fee and the L2 fee
where the L1 fee is the approximate cost of the batch submission of the
transaction (L1 gas price * L1 gas used) and the L2 fee is the
approximate cost of the execution on L2 taking into account congestion
(L2 gas price * L2 gas limit).

When either the L1 or L2 gas price is volatile, it can result in the
quote that the user receives from `eth_estimateGas` to be rejected
as the fee that the Sequencer is expecting has gone up.

This PR adds logic to set a buffer in either direction of the current
price that will allow the sequencer to still accept transactions within.

Two new config options are added:

- `--rollup.feethresholddown` or `ROLLUP_FEE_THRESHOLD_DOWN`
- `--rollup.feethresholdup` or `ROLLUP_FEE_THRESHOLD_UP`

Note that these config options are only useful for when running
in Sequencer mode, they are not useful for replicas/verifiers.
This is because the Sequencer is the only write node in the network.

These config options are interpreted as floating point numbers and are
multiplied against the current fee that the sequencer is expecting.
To allow for a downward buffer of 10% and an upward buffer of 100%,
use the options:

- `ROLLUP_FEE_THRESHOLD_DOWN=0.9`
- `ROLLUP_FEE_THRESHOLD_UP=2`

This will allow for slight fee volatility downwards and prevent users
from excessively overpaying on fees accidentally.

This is a UX and profit tradeoff for the sequencer and can be exploited
by bots. If bots are consistently taking advantage of this, the max
threshold down will have to be calibrated to what the normal fee is
today.

Both config options are sanity checked in the `SyncService` constructor
and will result in errors if they are bad. The threshold down must
be less than 1 and the threshold up must be greater than 1.

feat: add workflow for running SNX test suite

chore(ci): Skip check files job on push events

Version Packages

Merge develop to master

ci(contracts): only run codecov if contracts package is modified

In order to skip running the CodeCov job when it isn't relevant, a new job was added to the ts-packges workflow. For a PR, this job identifies all the files modified between the base branch and the tip of the PR branch, and writes to a variable which can be used to decide if subsequent jobs should run.

Github Actions provides an easy method for achieving this at the level of a workflow, but not for specific jobs within a workflow.

Add op_exporter for sequencer metrics and health endpoint

l2geth: use `hexutil.Big` to prevent overflows

The `RollupClient` decodes the JSON from the DTL as a uint64,
this updates it to be a `big.Int`

Includes a test for the value field that ensures it does not overflow

Increase client_body_buffer_size for rpc-proxy

Added client_max_body_size to rpc-proxy config

feat[integration-tests]: make integration tests work against production networks

Version Packages

feat: update README to include info about rebasing

Added metric for sequencer health
Added Dockerfile for op_exporter;
Fixed Dockerfile path

fix: update CI to avoid running out of memory

Merge develop into master

fix: lint deploy-l2 folder in contracts

dtl: configurable gas price backend

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.

Merge pull request #1033 from ethereum-optimism/elena/op-790-add-linting-andor-formatting-to-solidity

Add contracts linting

which changes the way it parses errors from returndata
Fix provided by @smartcontracts