A prior patch segmented channel snapshots, and changed the underlying
data object type associated with ast_channel_snapshot_type stasis
messages. Prior to Asterisk 18 it was a type ast_channel_snapshot, but
now it type ast_channel_snapshot_update.

When publishing ast_channel_snapshot_type in pbx_realtime the
ast_channel_snapshot was being passed in as the message data
object. When a handler, expecting a data object type of
ast_channel_snapshot_update, dereferenced this value a crash
would occur.

This patch makes it so pbx_realtime now uses the expected type, and
channel snapshot publish method when publishing.

ASTERISK-29168 #close

Change-Id: I9a2cfa0ec285169317f4b9146e4027da8a4fe896