Support ALTER THING .. DEPENDS ON EXTENSION

 Support ALTER THING .. DEPENDS ON EXTENSION
 
 This introduces a new dependency type which marks an object as depending
 on an extension, such that if the extension is dropped, the object
 automatically goes away; and also, if the database is dumped, the object
 is included in the dump output.  Currently the grammar supports this for
 indexes, triggers, materialized views and functions only, although the
 utility code is generic so adding support for more object types is a
 matter of touching the parser rules only.
 
 Author: Abhijit Menon-Sen
 Reviewed-by: Alexander Korotkov, Álvaro Herrera
 Discussion: http://www.postgresql.org/message-id/20160115062649.GA5068@toroid.org