Serialize vectors without indices

[tjardoo]

This is a proof of concept to serialize a vector without indices in the query string user_ids[]=100&user_ids[]=101. The default is using indices user_ids[0]=100&user_ids[1]=101. The structure without the indices is used by ie. the OpenAI API.

I added a new method to_string_config(input, config) to be able to serialize with a custom configuration. The existing method to_string(input) will work the same as before and will use the default config (using indices).

The new config currently only has the field use_indices of type boolean - but can be expanded later if needed.

I'm not really a fan of using two different methods, do you have any suggestion on how you would like this to work? @samscott89
As there is no standard for this type of serialization - if you do not want to support this, feel free to close it. Thanks.

let params = QueryParams {
    id: 42,
    user_ids: vec![1, 2, 3, 4],
};

let config = SerializerConfig { use_indices: false };

assert_eq!(
    serde_qs::to_string_config(&params, config).unwrap(),
    "\
    id=42&user_ids[]=1&user_ids[]=2&\
    user_ids[]=3&user_ids[]=4"
);

[tjardoo]

Resolves #80