allow setting sql dialect

[mdsumner]

[mdsumner]

this is more important now, as it is the pivot of what a real DB is vs OGRSQL, I think

[mdsumner]

old notes ... see the supported dialects stuff above, Nyall updated drivers to be able to expose what dialect/s they can use (see #175)

possible dialect values

• None - what you get depends on the drivers
• OGRSQL GDAL's virtual table mechanism
• SQLITE enabled by in-built SQLite/Spatiallite (GPKG, SQLite - opt out and use virtual table mech with INDIRECT_SQLITE)
• INDIRECT_SQLITE (use GDAL's SQLITE) https://gdal.org/user/sql_sqlite_dialect.html
• MONGODB for passing Mongo serialized commands https://gdal.org/drivers/vector/mongodbv3.html#executesql-interface
• ES for passing seralized Elasticsearch filter https://gdal.org/drivers/vector/elasticsearch.html#executesql-interface
• DEBUG - for autotest only

[mdsumner]

ok, PR incoming