Make it easier to provide context to fullstack

[ealmloff]

This PR changes the context function in the launch builder to provide context throughout the fullstack application including in server functions. You can now consume context in server functions with an extractor:

LaunchBuilder::new().with_context(1234567890u32).launch(app);

#[server]
async fn get_server_data() -> Result<String, ServerFnError> {
    let FromContext(context): FromContext<u32> = extract().await?;
    let text = reqwest::get("https://httpbin.org/ip").await?.text().await?;
    Ok(format!("context {context} Your IP address is: {text} 🌎"))
}

Closes #2131
Closes #1688

[scullionw]

how would .with_context be used to pass resources that require async to be built?

like if we want to store a database connection? (let conn = connect_to_db().await)

something like:

LaunchBuilder::new().with_context_future(async { connect_to_db().await }).launch(app);

?