Improve PostgresDialect.discoverDataCollections to reduce the start time of Postgres CDC

@Override
public Map<TableId, TableChange> discoverDataCollectionSchemas(JdbcSourceConfig sourceConfig) {
    final List<TableId> capturedTableIds = discoverDataCollections(sourceConfig);

    try (JdbcConnection jdbc = openJdbcConnection(sourceConfig)) {
        // fetch table schemas
        Map<TableId, TableChange> tableSchemas = new HashMap<>();
        for (TableId tableId : capturedTableIds) {
            TableChange tableSchema = queryTableSchema(jdbc, tableId);
            tableSchemas.put(tableId, tableSchema);
        }
        return tableSchemas;
    } catch (Exception e) {
        throw new FlinkRuntimeException(
                "Error to discover table schemas: " + e.getMessage(), e);
    }
}