Use 0x gas api instead of eth gas station api [TKR-502] (#532)

* Use 0x gas api instead of eth gas station api

* Add integration test for `ProtocolFeeUtils`

* Update CHANGELOG.json

packages/asset-swapper/test/protocol_fee_utils_test.ts

@@ -0,0 +1,45 @@
+import * as chai from 'chai';
+import 'mocha';
+import { rest } from 'msw';
+import { setupServer } from 'msw/node';
+
+import { ProtocolFeeUtils } from '../src';
+
+import { chaiSetup } from './utils/chai_setup';
+chaiSetup.configure();
+const expect = chai.expect;
+
+const server = setupServer(
+    rest.get('https://mock-0x-gas-api.org/median', (_req, res, ctx) => {
+        return res(
+            ctx.json({
+                result: {
+                    source: 'MEDIAN',
+                    timestamp: 1659386474,
+                    instant: 22000000000,
+                    fast: 18848500000,
+                    standard: 14765010000,
+                    low: 13265000000,
+                },
+            }),
+        );
+    }),
+);
+
+describe('ProtocolFeeUtils', () => {
+    describe('getGasPriceEstimationOrThrowAsync', () => {
+        beforeEach(() => {
+            server.listen();
+        });
+
+        afterEach(() => {
+            server.close();
+        });
+
+        it('parses fast gas price response correctly', async () => {
+            const utils = ProtocolFeeUtils.getInstance(420000, 'https://mock-0x-gas-api.org/median');
+            const gasPrice = await utils.getGasPriceEstimationOrThrowAsync();
+            expect(gasPrice.toNumber()).to.eq(18848500000);
+        });
+    });
+});