feat: Add optional Tower service integration to API clients

[joshrotenberg]

Overview

Adds optional tower::Service implementation to redis-cloud and redis-enterprise clients via the tower-integration feature flag, enabling Tower middleware composition without forcing dependencies on library consumers.

Changes

redis-cloud

• ✅ Added tower as optional dependency with tower-integration feature
• ✅ Implemented tower::Service for CloudClient
• ✅ Added tower_support module with ApiRequest/ApiResponse types
• ✅ Added into_service() helper method
• ✅ Updated README with Tower integration examples

redis-enterprise

• ✅ Added tower as optional dependency with tower-integration feature
• ✅ Implemented tower::Service for EnterpriseClient
• ✅ Added tower_support module with ApiRequest/ApiResponse types
• ✅ Added into_service() helper method
• ✅ Updated README with Tower integration examples

Usage Example

use redis_cloud::CloudClient;
use redis_cloud::tower_support::ApiRequest;
use tower::ServiceExt;

let client = CloudClient::builder()
    .api_key("key")
    .api_secret("secret")
    .build()?;

// Convert to Tower service
let mut service = client.into_service();

// Use with Tower middleware
let response = service
    .oneshot(ApiRequest::get("/subscriptions"))
    .await?;

Benefits

• ✅ Opt-in dependencies - Tower only required if feature enabled
• ✅ Zero overhead - No impact when feature is disabled
• ✅ Ecosystem compatible - Works with tower-resilience, tower-http, etc.
• ✅ Future-proof - Foundation for resilience patterns (issue feat: Add tower-resilience patterns to CLI #446)
• ✅ Library-first - Available to all consumers, not just CLI

Testing

• ✅ Compiles with feature enabled
• ✅ Compiles without feature (no regression)
• ✅ Passes cargo clippy --all-features
• ✅ Passes cargo fmt --check

Related Issues

Closes #445
Prerequisite for #446 (CLI resilience integration)

Breaking Changes

None - This is purely additive with an opt-in feature flag.