Migrate Request APIs to async

[inancgumus]

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.

Tasks

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1. APIs Migrated:
2. Release Notes:
3. Doc Updates:
4. Type definitions Updates