res_pjsip: Add handling for incoming unsolicited MWI NOTIFY

A new endpoint parameter "incoming_mwi_mailbox" allows Asterisk to
receive unsolicited MWI NOTIFY requests and make them available to
other modules via the stasis message bus.

res_pjsip_pubsub has a new handler "pubsub_on_rx_mwi_notify_request"
that parses a simple-message-summary body and, if
endpoint->incoming_mwi_account is set, calls ast_publish_mwi_state
with the voice-message counts from the message.

Change-Id: I08bae3d16e77af48fcccc2c936acce8fc0ef0f3c

contrib/ast-db-manage/config/versions/a1698e8bb9c5_add_incoming_mwi_mailbox.py

@@ -0,0 +1,21 @@
+"""add_incoming_mwi_mailbox
+
+Revision ID: a1698e8bb9c5
+Revises: b83645976fdd
+Create Date: 2017-09-08 13:45:18.937571
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = 'a1698e8bb9c5'
+down_revision = 'b83645976fdd'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.add_column('ps_endpoints', sa.Column('incoming_mwi_mailbox', sa.String(40)))
+
+def downgrade():
+    op.drop_column('ps_endpoints', 'incoming_mwi_mailbox')