Add read_from_replicas method and environment variable support for Redis cluster configuration

[fruscianteee]

Overview

This PR introduces the following changes:

1. Added the read_from_replicas method to the Config struct for easier configuration.
   • The read_from_replicas method allows enabling read operations from Redis replica nodes in a more intuitive way.
2. Enabled reading the REDIS_CLUSTER__READ_FROM_REPLICAS=true environment variable for configuration.
   • The configuration now supports environment variables such as REDIS_CLUSTER__READ_FROM_REPLICAS for enabling or disabling reading from replicas.

Changes

• Implemented the read_from_replicas method to simplify the process of enabling replica reads.
• Updated the configuration handling to support reading environment variables, specifically REDIS_CLUSTER__READ_FROM_REPLICAS.

Testing

• Added a test to ensure that Redis write operations work correctly when read_from_replicas is enabled.
• Skipped testing whether read commands are actually sent to replicas, as this behavior is managed internally by redis-rs, and we rely on its implementation for this verification.

Issue

This PR addresses issue #350.

[bikeshedder]

Could you please look into the clippy error and remove the superfluous #[must_use]. Actually I would prefer for the read_from_replicas method to be removed. Writing cfg.read_from_replicas = true is just as easy as calling a cfg.read_from_replicas() method.

[bikeshedder]

Is there a reason to keep the read_from_replicas() method? I'd rather remove it if it doesn't add any value. It just adds multiple ways to do the same thing.

[fruscianteee]

Thank you for your comment. The reason I implemented the read_from_replicas method was that I thought it might be easier to understand if the enabling method worked similarly to how it's done in redis-rs. However, as you pointed out, it would be better to keep things simpler by removing the method. I will proceed with that change.

[fruscianteee]

(I made the must_use change before receiving your comment, which might have caused some confusion, making it seem like I wouldn't address the read_from_replicas change. I apologize for that.)

[bikeshedder]

Could you also add this to the CHANGELOG.md of the deadpool-redis crate? That'd be awesome. 🥳

I was about to make a deadpool-redis release. As this changes a public structure it needs to be a "breaking release" even though I don't think anyone uses the cluster config directly.

[fruscianteee]

OK! I will also add the changes to the CHANGELOG.md, so please wait a moment.

[bikeshedder]

You changed the r2d2/CHANGELOG.md by accident. That should've been added to the redis/CHANGELOG.md instead. 🙈

[fruscianteee]

Oh no! I’m sorry! I’ve fixed it now!