new: [postgres] new ``init-sql`` option in relation to init any database with given sql

postgres/hooks/postgres_database-relation-joined

@@ -52,6 +52,10 @@ ensure_db_docker_running
 ## XXXvlab: should send all these into only one docker...
 if ! db_has_database "$DBNAME"; then
     db_create "$DBNAME" || exit 1
+
+    if sql=$(relation-get init-sql); then
+        ddb "$DBNAME" > /dev/null < <(e "$sql") || exit 1
+    fi
 fi
 if [ "${#extensions[@]}" -gt 0 ]; then
     db_install_extensions "$DBNAME" "${extensions[@]}" || exit 1