AS API: Allow passing headers when making GraphQL requests

[ospencer]

This may also need to be implemented for other languages. Headers can be passed the same way as HTTP handles headers.

[cohix]

@ospencer is this for headers other than auth?

[ospencer]

I hadn't really thought about it—this was mostly for one-off auth (if you couldn't use Atmo's auth handling), though there might be some use cases where it'd be useful for other headers (analytics, etc).