MINOR: payload: add sample fetch for TLS ALPN

MINOR: payload: add sample fetch for TLS ALPN

Application-Layer Protocol Negotiation (ALPN, RFC7301) is a TLS
extension which allows a client to present a preference for which
protocols it wishes to connect to, when a single port supports multiple
multiple application protocols.
It allows a transparent proxy to take a decision based on the beginning
of an SSL/TLS stream without deciphering it.

The new fetch "req.ssl_alpn" extracts the ALPN protocol names that may
be present in the ClientHello message.