channels/pjsip/dialplan_functions: Add an option for extracting the SIP call-id

This patch adds a new option to the CHANNEL function that allows for the
extraction of the SIP call-id. It is used in conjunction with the 'pjsip'
option, and will return the Call-ID of the INVITE request that established
the PJSIP channel.

ASTERISK-25352

Change-Id: I278d1f8bcfe3a53c5aa1dadebc14e92b0abd476a