Documentation on how to make custom extractors

[sergiomeneses]

Is your feature request related to a problem? Please describe.
I want to create a custom extrators for my app.

Describe the solution you'd like
Add an example of a extrator that access state.

Describe alternatives you've considered
Just docs

Additional context
I need to create my custom extrators but i found so far the following issues:

1. When i implement FromMessageParts (or any of these handler traits), i can't get the json_serde::Value andbytes::Bytes types (you should export these), i currently using sqlx::types::JsonValue andaxum::body::Bytes.
2. If i need my custom extrator to access socket state, the only solution at the moment is to create the state from the current message (also for other type handlers) i.e. State::<T>::from_message_parts(s, v, p, ack_id) which returns Result<Self, StateNotFound<T>>, so i have consum the result and covert to option using ok(), and finally unwrap() the option, which is safe in my context but i think we need a better way to do this kind of extractors (axum gets the state as parameter in the from_request_parts function).
3. Also we need a way to multiple extrator like axums does i.e `Data((one, two, three)): Data<(TypeOne, TypeTwo, TypeThree)>

[Totodore]

I agree that custom extractor implementations lack examples and docs.

However I'm curious about what you are trying to do, because if you want to access the axum state you simply have to make a shared state and add it to both axum and socketioxide with with_state and then you can use the State extractor provided by socketioxide.
But maybe I didn't understand you original issue.