Add support for REINDEX in event triggers

Add support for REINDEX in event triggers

This commit adds support for REINDEX in event triggers, making this
command react for the events ddl_command_start and ddl_command_end.  The
indexes rebuilt are collected with the ReindexStmt emitted by the
caller, for the concurrent and non-concurrent paths.

Thanks to that, it is possible to know a full list of the indexes that a
single REINDEX command has worked on.

Author: Garrett Thornburg, Jian He
Reviewed-by: Jim Jones, Michael Paquier
Discussion: https://postgr.es/m/CAEEqfk5bm32G7sbhzHbES9WejD8O8DCEOaLkxoBP7HNWxjPpvg@mail.gmail.com