Add sipp-sendfax.xml and spandspflow2pcap.py to contrib/scripts.

The spandspflow2pcap.py creates pcap files from fax.log files, generated
through 'fax set debug on' when receiving a fax. An example fax.log is
included as spandspflow2pcap.log.

The sipp-sendfax.xml SIPp scenario can be used to replay that fax with a
recent version of SIPp.

ASTERISK-25660 #close

Change-Id: I4de8f28b084055b482ab8a5b28d28b605b0ed526