provide fts_flatcurve

Issue description

Right now, SNM only provides FTS via fts_xapian, which is pretty memory intensive.

According to https://github.com/slusarz/dovecot-fts-flatcurve/issues/4#issuecomment-902425597, fts_flatcurve seems to be a better fit, and, since April 2022, ended up in dovecot core.

It'd be nice if SNM would provide the option to use that, probably even recommend it as a default.