Add vtable and methods for to_json and to_ami for Stasis messages

When a Stasis message type is defined in a loadable module, handling
those messages for AMI and res_stasis events can be cumbersome.

This patch adds a vtable to stasis_message_type, with to_ami and
to_json virtual functions. These allow messages to be handled
abstractly without putting module-specific code in core.

As an example, the VarSet AMI event was refactored to use the to_ami
virtual function.

(closes issue ASTERISK-21817)
Review: https://reviewboard.asterisk.org/r/2579/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@391403 f38db490-d61c-443f-a65b-d21fe96a405b