res_pjsip_notify.c: enable in-dialog NOTIFY

This patch adds support to send in-dialog SIP NOTIFY commands on
chan_pjsip channels, similar to the functionality recently added
for chan_sip (ASTERISK_27461).

This extends res_pjsip_notify to allow for in-dialog messages.

ASTERISK-27697

Change-Id: If7f3151a6d633e414d5dc319d5efc1443c43dd29