feat(api): Implement eth_maxPriorityFeePerGas (#3135)

## What ❔

Fixes #13 
Implements `eth_maxPriorityFeePerGas` method.

## Why ❔

This method is a de-facto standard now, and SDKs (e.g. viem) can use it
assuming that it's supported.
Even given that we're not really using EIP1559, we should still support
it and return 0.

## Checklist

<!-- Check your PR fulfills the following items. -->
<!-- For draft PRs check the boxes as you complete them. -->

- [ ] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [ ] Tests for the changes have been added / updated.
- [ ] Documentation comments have been added / updated.
- [ ] Code has been formatted via `zkstack dev fmt` and `zkstack dev
lint`.

core/lib/web3_decl/src/namespaces/eth.rs

@@ -185,6 +185,9 @@ pub trait EthNamespace {
         newest_block: BlockNumber,
         reward_percentiles: Vec<f32>,
     ) -> RpcResult<FeeHistory>;
+
+    #[method(name = "maxPriorityFeePerGas")]
+    async fn max_priority_fee_per_gas(&self) -> RpcResult<U256>;
 }
 
 #[cfg(feature = "server")]

core/node/api_server/src/web3/backend_jsonrpsee/namespaces/eth.rs

@@ -268,4 +268,8 @@ impl EthNamespaceServer for EthNamespace {
             .await
             .map_err(|err| self.current_method().map_err(err))
     }
+
+    async fn max_priority_fee_per_gas(&self) -> RpcResult<U256> {
+        Ok(self.max_priority_fee_per_gas_impl())
+    }
 }

core/node/api_server/src/web3/namespaces/eth.rs

@@ -863,6 +863,11 @@ impl EthNamespace {
             }
         })
     }
+
+    pub fn max_priority_fee_per_gas_impl(&self) -> U256 {
+        // ZKsync does not require priority fee.
+        0u64.into()
+    }
 }
 
 // Bogus methods.

core/tests/ts-integration/tests/api/web3.test.ts

@@ -189,7 +189,8 @@ describe('web3 API compatibility tests', () => {
         ['eth_getCompilers', [], []],
         ['eth_hashrate', [], '0x0'],
         ['eth_mining', [], false],
-        ['eth_getUncleCountByBlockNumber', ['0x0'], '0x0']
+        ['eth_getUncleCountByBlockNumber', ['0x0'], '0x0'],
+        ['eth_maxPriorityFeePerGas', [], '0x0']
     ])('Should test bogus web3 methods (%s)', async (method: string, input: string[], output: string) => {
         await expect(alice.provider.send(method, input)).resolves.toEqual(output);
     });
@@ -271,7 +272,8 @@ describe('web3 API compatibility tests', () => {
 
         const eip1559ApiReceipt = await alice.provider.getTransaction(eip1559Tx.hash);
         expect(eip1559ApiReceipt.maxFeePerGas).toEqual(eip1559Tx.maxFeePerGas!);
-        expect(eip1559ApiReceipt.maxPriorityFeePerGas).toEqual(eip1559Tx.maxPriorityFeePerGas!);
+        // `ethers` will use value provided by `eth_maxPriorityFeePerGas`, and we return 0 there.
+        expect(eip1559ApiReceipt.maxPriorityFeePerGas).toEqual(0n);
     });
 
     test('Should test getFilterChanges for pending transactions', async () => {