[v0.3] pass `request-options` to `outgoing-handler` by `borrow`

[pchickey]

In 0.2, outgoing-handler gets passed an option<request-options>:

  @since(version = 0.2.0)
  handle: func(
    request: outgoing-request,
    options: option<request-options>
  ) -> result<future-incoming-response, error-code>;

It would be nice if this was instead an option<borrow<request-options>> so that the same options could be reused across many requests.

We can't change this in 0.2 because it would be a breaking change, but we should take the opportunity to fix it in 0.3.