Add Response.trailers and write trailing fields to it

Rename Response.headers to Response.fields

RFC-9110 Section 5 specifically refers to the non-body message values as "fields" instead of the common name "headers" which makes sense as they can appear in both header and trailer sections of the message.

As such Response.headers should be renamed to Response.fields.

Add Response.trailers and write trailing fields to it

Update: RFC-9110 Section 6.5 explicitly states header and trailer fields should be stored separately (and that header fields and trailer fields are referred to as headers and trailers) so Response.headers should remain, with a new Response.trailers object for trailing fields.