res_pjsip_mwi: Send unsolicited MWI NOTIFY on startup and when endpoint registers.

Currently the res_pjsip_mwi module only sends an unsolicited MWI NOTIFY upon
a mailbox state change (such as a new message being left, or one being deleted).
In practice this is not sufficient to keep clients aware of the current MWI status.

This change makes the module send unsolicited MWI NOTIFY on startup so that
clients are guaranteed to have the most up to date MWI information. It also makes
clients receive an unsolicited MWI NOTIFY upon registration so if they are unaware
of the current MWI status they receive it.

ASTERISK-24982 #close
Reported by: Joshua Colp

Change-Id: I043f20230227e91218f18a82c7d5bb2aa62b1d58