res_pjsip_pubsub: Add new pubsub module capabilities. (#82)

res_pjsip_pubsub: Add new pubsub module capabilities. (#82)

The existing res_pjsip_pubsub APIs are somewhat limited in
what they can do. This adds a few API extensions that make
it possible for PJSIP pubsub modules to implement richer
features than is currently possible.

* Allow pubsub modules to get a handle to pjsip_rx_data on subscription
* Allow pubsub modules to run a callback when a subscription is renewed
* Allow pubsub modules to run a callback for outgoing NOTIFYs, with
  a handle to the tdata, so that modules can append their own headers
  to the NOTIFYs

This change does not add any features directly, but makes possible
several new features that will be added in future changes.

Resolves: #81
ASTERISK-30485 #close

Master-Only: True