res_rtp_asterisk: make it possible to remove SOFTWARE attribute

By default Asterisk reports the PJSIP version in a SOFTWARE attribute
of every STUN packet it sends. This may not be desired in a production
environment, and RFC5389 recommends making the use of the SOFTWARE
attribute a configurable option:

https://datatracker.ietf.org/doc/html/rfc5389#section-16.1.2

This patch adds a `stun_software_attribute` yes/no option to make it
possible to omit the SOFTWARE attribute from STUN packets.

ASTERISK-29434

Change-Id: Id3f2b1dd9584536ebb3a1d7e8395fd8b3e46860b