feat(core,legacy-scripting-runner): allow GET requests to have body

[eliangcs]

Adds a hidden option allowGetBody to z.request(). When allowGetBody is true, z.request won't delete the body for the GET request. This is to add support for WB's ability to send GET requests with a body.

This has to be done with a nasty hack because node-fetch explicitly forbids us to send a GET request with a body (see also whatwg/fetch#551). Not an ideal solution, but it gets the job done.

[xavdid]

woof, this one is gross. The only improvement i could think to make is restricting this to apps w/ the legacy property so that there aren't any proper CLI apps that take advantage of this. Not a blocker if that's not trivial to add though.

Thanks!

[eliangcs]

@xavdid I think this "feature" can still be useful for CLI apps. We probably don't want to encourage this usage—that's why I made it a "hidden" option. But there are APIs like ElasticSearch that requires users to GET with body. So we should at least provide an option for developers to do it.