realtime: Create extensions in alembic ast-db-manage contribution

When the alembic scripts were written for creating Asterisk
realtime databases the extensions table for dialplan wasn't
included.  This update creates the extensions table.

(closes issue ASTERISK-22815)
Reported by: Zone Conkle
Review: https://reviewboard.asterisk.org/r/3064/
........

Merged revisions 403713 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403714 65c4cc65-6c06-0410-ace0-fbb531ad65f3