Migrate Request APIs to async

[inancgumus]

### Tasks
- [ ] #1348
- [ ] #1349
- [ ] https://github.com/grafana/xk6-browser/pull/1354
- [ ] https://github.com/grafana/xk6-browser/pull/1358
- [ ] https://github.com/grafana/xk6-browser/pull/1363
- [ ] grafana/k6-docs#1603
- [ ] https://github.com/grafana/k6-docs/pull/1614
- [ ] https://github.com/grafana/k6-DefinitelyTyped/pull/55
- [x] Release Notes:

---

What

Migrate the following APIs according to the How section in #428:

Class	Method	Current k6 Return	PW return
Request	allHeaders	Record<string,string>	Promise<{[key:string]:string;}>
Request	headerValue	string|null	Promise<null|string>
Request	headersArray	Array<{name:string;value:string}>	Promise<Array<{name:string;value:string;}>>
Request	postData	string	null|string
Request	resourceType	ResourceType	string
Request	response	Response|null	Promise<null|Response>
Request	size	{body:number;headers:number}	not found in Playwright
Request	timing	ResourceTiming	{startTime:number;domainLookupStart:number;domainLookupEnd:number;connectStart:number;secureConnectionStart:number;connectEnd:number;requestStart:number;responseStart:number;responseEnd:number;}

For details on the Why and How, check the overarching issue in #428.

[allansson]

The type definitions for this are already async.

[inancgumus]

@allansson Looks like they (Response and Request) are not async? Am I missing something?

https://github.com/grafana/k6-DefinitelyTyped/blob/134104e2c91b1c5d919d700afb79980165039840/types/k6/experimental/browser.d.ts#L3778

[allansson]

Let me double-check. 🤔

[allansson]

My bad. I misread the expected return types of the tests. Will update these and the response types.