Add dynamic credentials providers for cloud IAM support

[richardhenry]

This PR adds the ability to provide credentials dynamically which is useful for cloud IAM authentication where the token/password expires. Otherwise rustis will attempt to reconnect using expired credentials.

Here's an example of how this works:

use rustis::{
    client::{Client, Credentials, CredentialsContext, WithCredentialsProvider},
    commands::ConnectionCommands,
    Result,
};

async fn fetch_iam_token(_ctx: &CredentialsContext) -> Result<String> {
    todo!("Call your cloud SDK or metadata service here")
}

#[tokio::main]
async fn main() -> Result<()> {
    let client = Client::connect(
        "rediss://cache.example.com:6379".with_credentials_provider(|ctx| async move {
            let token = fetch_iam_token(&ctx).await?;
            Ok(Credentials::for_default_user(token))
        }),
    )
    .await?;

    let _: String = client.ping("hello").await?;
    Ok(())
}

Intended to be compatible with sentinel, pooling, and backwards compatible (only additions to existing API).

Includes tests (green) and docs.

I was originally patching around this in my application, but it made things a lot more complicated. Implementing this at the library level is a lot less code than at the application level (and can be well tested, etc).

Responsible AI usage: Codex helped me make some of these changes and write the docs, but I carefully reviewed everything and am using this fork already in my running app.