Custom Headers for Endpoints

[drewag]

The ideas was initially discussed here.

Even outside of auth, certain endpoints may want to include custom headers.

The big question is whether to think of those headers as additional input or something else? Should they simply be an optional endpoint property of type [(String,String?)] or should we somehow use Coding? Should headers be one of the input formats? What if we want a combination of headers, url query, and body content?