net/asterisk20: Add MP3 Decoder Code for Format_MP3 support

Format_MP3, used to play MP3 encoded voicemail and music on hold, relies
on decoder library files that are not included in the main Asterisk
distribution.

PR: 292715
Tested by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
Approved by: Oleksandr Kryvulia <o.kryvulia@flex-it.com.ua>
Co-authored-by: Justin T. Gibbs <gibbs@FreeBSD.org>