HTTP/3 request and response message exchanges allow multiple HEADERS
frames to be sent, which allows for trailer sections or non-final
responses. Previously, quiche has only supported generating a single
HEADERS frame per request or response, and didn't support receiving
them.

This change adds support for sending and receiving multiple HEADERS
frames on a request stream. It enforces the sequencing rules defined in
https://www.rfc-editor.org/rfc/rfc9114.html#section-4.1.
Sending additional HEADERS is done using the new
`send_additional_headers()` method. On the receive side, rather than add
a new Event enum variant for these additional uses of HEADERS, this
change reuses Event::Headers. Previosuly, this event included a
`has_body` boolean, which has been renamed to `more_frames` to better
align with how HTTP/3 streams work.