http.send should support passing headers

[redbaron]

It would be handy if http.send supported passing headers.

[tsandall]

+1. The work to do this should be relatively straightforward. The only thing that I'm concerned about is if those headers are used to communicate secrets. If that's the case, we don't want to encourage people to hardcode secrets into their policies.

Work required:

• Update topdown/http.go#executeHTTPRequest to pull headers out of the input object provided to http.send and construct the http.Request headers.

Related: #420

[BenderScript]

I will take a stab at this one

[kichristensen]

Isn't this closed by 3e0fe4b?