feat: congestion-aware gas price oracle

[jonastheis]

1. Purpose or design rationale of this PR

When there are few transactions, the gas price oracle (GPO) can be skewed by a handful of high-price transactions easily, after which most clients would start using the higher gas price reported by GPO, even though their transaction would be processed with a lower gas price. Thus starting a upward price cycle.

This PR addresses this issue by taking into account current "congestion" levels as perceived by the txpool (run geth with -gpo.congestionthreshold) and returns a configurable minimum price (pre EIP 1559, takes -txpool.pricelimit) and a minimum tip of 1 Wei post EIP 1559.

2. PR title

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• feat: A new feature
• fix: A bug fix
• perf: A code change that improves performance
• refactor: A code change that doesn't fix a bug, or add a feature, or improves performance
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• test: Adding missing tests or correcting existing tests

3. Deployment tag versioning

Has the version in params/version.go been updated?

• This PR doesn't involve a new deployment, git tag, docker image tag, and it doesn't affect traces
• Yes

4. Breaking change label

Does this PR have the breaking-change label?

• This PR is not a breaking change
• Yes

[colinlyguo]

LGTM

[colinlyguo]

fyi, tested with --gpo.congestionthreshold 1000 before and after Curie transition, the GPO works as expected:

before Curie (0.15 gwei, which is equal to --txpool.pricelimit "$L2GETH_MIN_GAS_PRICE", L2GETH_MIN_GAS_PRICE="150000000" # 0.15 gwei):

eth_maxPriorityFeePerGas: 0.15 gwei.
eth_gasPrice: 0.15 gwei.

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_maxPriorityFeePerGas","id":1}' -H "Content-Type: application/json" http://10.6.11.134:8545 | jq
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x8f0d180"
}

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_gasPrice","id":1}' -H "Content-Type: application/json" http://10.6.11.134:8545 | jq
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x8f0d180"
}

After Curie (when tx pool is not congested):

eth_maxPriorityFeePerGas: 1 wei.
eth_gasPrice: 0.15 gwei + 1 wei.

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_maxPriorityFeePerGas","id":1}' -H "Content-Type: application/json" http://10.6.11.134:8545 | jq
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x1"
}

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_gasPrice","id":1}' -H "Content-Type: application/json" http://10.6.11.134:8545 | jq
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x8f0d181"
}