Support HTTP Headers in gatsby serve

[techfg]

Background

Neither gatsby serve nor gatsby develop emit headers (from the new headers configuration property). Given headers is now a core feature of Gatsby, these headers should be emitted at least in gatsby serve and possibly gatsby develop (if reasonable).

Approaches

While I haven't tested, it appears that for DSG & SSR pages, serve will emit headers (via serverDataHeaders). Similar approach can be leveraged for SSG along with ensuring that all headers are property emitted for DSG & SSR.

Thoughts/input welcome!