s_server: Support reading HTTP request from early data

This would be useful when testing with browsers / downloaders which
support 0-RTT only through HTTP.

Signed-off-by: Daiki Ueno <dueno@redhat.com>

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16055)