[Feature]: Support of postData for firefox

[abarreau]

Feature description

Hello,

While trying to fetch postData on a request using firefox, I came accross an "UnsupportedOperation" exception:

(node:20514) UnhandledPromiseRejectionWarning: UnsupportedOperation
    at BidiHTTPRequest.postData (<redacted>)

The following documentation page does not mention that postData was not supported: https://pptr.dev/webdriver-bidi#puppeteer-features-not-supported-over-webdriver-bidi

Do you have plan to support it?

Thanks for your help.

[OrKoN]

In short, yes, it is planned. The timeframe depends on the spec change in w3c/webdriver-bidi#1011 landing and the support being available in Firefox. cc @juliandescottes @sadym-chromium