Updates and fixes to HttpSensitivityGenerator (#1755)

* Simplify sensitivity detection logic by assuming bindings only exist at top level input/output.

* Fix greedy label logic - they can now exist alongside of normal labels.

* Fix `httpPrefixHeader` false positive.

* Export `RequestFmt` and `ResponseFmt` types alongside the closures.

* More extensive use of `Writable` to improve readability.