add "rds_csv_quote_string" command

can add one command to control this module to quote the string field or not.
so if using this command and command "rds_csv_field_name_header", we can server binary data from RDS upstream (eg: image).

location ~ '/cats/image/([0-9]+)' {
    drizzle_query 'select img from cat_image where id=$1 limit 1';
    drizzle_pass backend;
    rds_csv on;
    rds_csv_quote_string off;
    rds_csv_field_name_header off;
    rds_csv_content_type 'image/jpeg';
}

i have write simple code on the fork https://github.com/lloydzhou/rds-csv-nginx-module/commit/5c2649352fc20d24b26604701118ec7673408877.

Start with the fork commit linked in the issue and the existing rds_csv and rds_csv_field_name_header directives in this module. Verify that rds_csv_quote_string off with the field-name header disabled can return binary RDS data, and confirm the example's image content type behavior before adding coverage.