Add core events for adding columns to MySQL and Postgres search backends

These events will allow adding new columns (besides post_subject and post_text) to the search index.
The native search index doesn't need a new event as it manually creates the index, so the core.search_native_index_before event can be used. The Sphinx backend already has the core.search_sphinx_modify_config_data event that allows doing this. This PR aims at making it possible for MySQL and Postgres as well.