feat: allow independent retrieval of trailers

This commit changes wasi:http/types#body.finish to forward the
optionally present trailers, rather than retrieve them with the
possibility of generating an error along the way.

With this change, callers of `body.finish` will have to retreive
trailers on their own, which *may* produce an efficiency gain in the
case where trailers are present but not needed, and avoiding work of
checking for trailers in the first place.

Signed-off-by: Victor Adossi <[email protected]>

wit-0.3.0-draft/types.wit

@@ -247,7 +247,7 @@ interface types {
     /// are fully transmitted.
     new: static func(
       %stream: stream<u8>,
-      trailers: option<future<trailers>>
+      trailers: future<option<trailers>>
     ) -> tuple<body, future<result<_, error-code>>>;
 
     /// Returns the contents of the body, as a stream of bytes.
@@ -260,9 +260,10 @@ interface types {
     /// The returned future resolves to success if body is closed.
     %stream: func() -> result<tuple<stream<u8>>, future<result<_, error-code>>>;
 
-    /// Takes ownership of `body`, and returns a `trailers`.  This function will
-    /// trap if a `stream` child is still alive.
-    finish: static func(this: body) -> result<option<trailers>, error-code>;
+    /// Takes ownership of `body`, and returns an unresolved optional `trailers`.
+    ///
+    /// This function will trap if a `stream` child is still alive.
+    finish: static func(this: body) -> future<option<trailers>>;
   }
 
   /// Represents an HTTP Request.