Feature: Support fulltext indexes

HarbourBridge's current treatment of fulltext indexes (mysql) and equivalent functionality in postgres (GIN and GiST indexes) is partial and inconsistent.

With the mysql driver, fulltext indexes are mapped to regular Spanner indexes.

With the mysqldump driver, fulltext indexes are silently ignored/dropped.

We should:
- figure out how we want to handle fulltext indexes (only realistic options at present are (a) drop them and document this in the report, or (b) map them to regular indexes, which probably isn't useful)
- implement this consistently across dump and direct drivers (and for both mysql and postgres)
- update this treatment as new features are available on Spanner