Get underlying database connection
Examples
if (FALSE) {
con <- DBI::dbConnect(duckdb::duckdb(), dbdir = eunomia_dir())
cdm <- cdm_from_con(con = con, cdm_name = "Eunomia",
cdm_schema = "main", write_schema = "main")
cdmCon(cdm)
DBI::dbDisconnect(con)
}