res_pjsip_exten_state: Create PUBLISH messages.

Create PUBLISH messages to update a third party when an extension state
changes because of either a device or presence state change.

A configuration example:

[exten-state-publisher]
type=outbound-publish
server_uri=sip:instance1@172.16.10.2
event=presence
; Optional regex for context filtering, if specified only extension state
; for contexts matching the regex will cause a PUBLISH to be sent.
@context=^users
; Optional regex for extension filtering, if specified only extension
; state for extensions matching the regex will cause a PUBLISH to be sent.
@exten=^[0-9]*
; Required body type for the PUBLISH message.
;
; Supported values are:
; application/pidf+xml
; application/xpidf+xml
; application/cpim-pidf+xml
; application/dialog-info+xml (Planned support but not yet)
@body=application/pidf+xml

The '@' extended variables are used because the implementation can't
extend the outbound publish type as it is provided by the outbound publish
module.  That means you either have to use extended variables, or
implement some sort of custom extended variable thing in the outbound
publish module.  Another option would be to refactor that stuff to have an
option which specifies the use of an alternate implementation's
configuration and then have that passed to the implementation.  JColp
opted for the extended variables method originally.

ASTERISK-25972 #close

Change-Id: Ic0dab4022f5cf59302129483ed38398764ee3cca