webRequest block allows custom headers and method

This block lets you specify arbitrary http headers (as json) and select from a given list of http methods. 

Hopefully that list is comprehensive enough, there are a few others I decided to omit to avoid an unnecessarily long dropdown.

Alternative: use a text field instead of a dropdown for the method. I can be persuaded either way on that.

Having a webrequest block addresses the concerns in issue #468 around sending authentication via headers to use some external APIs.