feat: impl EthGasPrice RPC API

[hanabi1224]

Summary of changes

As part of #3640

Changes introduced in this pull request:

• Implement Filecoin.EthGasPrice RPC API
• Test coverage in forest-tool api compare

     Running `target/debug/forest-tool api compare /home/me/fr/snapshots/calibnet/forest_snapshot_calibnet_2023-12-07_height_1153690.forest.car.zst --filter EthGas`
| RPC Method           | Forest | Lotus |
|----------------------|--------|-------|
| Filecoin.EthGasPrice | Valid  | Valid |

Lotus code

func (a *EthModule) EthGasPrice(ctx context.Context) (ethtypes.EthBigInt, error) {
	// According to Geth's implementation, eth_gasPrice should return base + tip
	// Ref: https://github.com/ethereum/pm/issues/328#issuecomment-853234014

	ts := a.Chain.GetHeaviestTipSet()
	baseFee := ts.Blocks()[0].ParentBaseFee
	premium, err := a.EthMaxPriorityFeePerGas(ctx)
	if err != nil {
		return ethtypes.EthBigInt(big.Zero()), nil
	}
	gasPrice := big.Add(baseFee, big.Int(premium))
	return ethtypes.EthBigInt(gasPrice), nil
}

func (a *EthModule) EthMaxPriorityFeePerGas(ctx context.Context) (ethtypes.EthBigInt, error) {
	gasPremium, err := a.GasAPI.GasEstimateGasPremium(ctx, 0, builtinactors.SystemActorAddr, 10000, types.EmptyTSK)
	if err != nil {
		return ethtypes.EthBigInt(big.Zero()), err
	}
	return ethtypes.EthBigInt(gasPremium), nil
}

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[lemmih]

Maybe this should be hexify? And it should probably live in lotus_json/mod.rs. It'll definitely be used by other RPC methods as well.

[hanabi1224]

Fixed.