Add `REDIS_USERNAME` environment variable support

[burgerkorb731]

**Is your feature request related to a problem? Please describe. **
It seems like Outpost only supports connecting to Redis using the default user. This creates issues when connecting to managed Redis services (like AWS ElastiCache/Serverless Cache) that require authentication with non-default users.

Describe the solution you'd like
Add support for a REDIS_USERNAME environment variable to allow authentication with custom Redis users.

Describe alternatives you've considered
Currently, the only workaround is to:

1. Grant full permissions to the default user (on ~* +@all)
2. Use the default user's credentials
3. This violates principle of least privilege and security best practices for our organization.

Using REDIS_URL instead of individual fields, but this appears to not be supported based on the .env.example.

Additional context

Environment

• Outpost version: [latest]
• Redis service: AWS ElastiCache Serverless Cache (Valkey 8)
• Deployment: Kubernetes

[leggetter]

Initial Research: Redis Username Support Investigation

I investigated the claim that Outpost doesn't support REDIS_USERNAME. Summary:

Finding: The claim is valid

Outpost does not currently support REDIS_USERNAME for Redis authentication.

Evidence from codebase

1. Configuration structure:

   • RedisConfig in internal/config/config.go (lines 360-368) only includes: Host, Port, Password, Database, TLSEnabled, ClusterEnabled, and DevClusterHostOverride
   • No Username field exists in the configuration struct

2. Redis client implementation:

   • In internal/redis/redis.go, both createRegularClient() and createClusterClient() only set the Password field when creating connections
   • The underlying github.com/redis/go-redis/v9 library does support username authentication, but Outpost doesn't pass it through

3. No existing references:

   • A search of the codebase shows zero references to REDIS_USERNAME

Why the Azure example works without username

The Azure Managed Redis example in examples/azure/ works because it uses a different authentication model:

1. Access Keys Authentication: The Azure setup (see examples/azure/dependencies.sh line 172) creates Redis with --access-keys-auth "Enabled", which enables password-only authentication using an implicit default user.

2. Different Redis services, different auth models:

   • Azure Managed Redis Enterprise (with access-keys-auth): Uses password-only authentication with a default user
   • AWS ElastiCache Serverless Cache (Valkey 8): Uses Redis ACL authentication, which requires explicit username + password

The core issue

AWS ElastiCache Serverless Cache requires Redis ACL authentication with explicit usernames. Outpost cannot connect to such instances because:

• It only supports password-only authentication
• It doesn't support the Username field required for ACL-based authentication

Recommendation

To support Redis instances that require ACL authentication (like AWS ElastiCache Serverless), Outpost would need to:

1. Add a Username field to RedisConfig in internal/config/config.go
2. Add a Username field to redis.RedisConfig in internal/redis/config.go
3. Pass the username to both r.Options and r.ClusterOptions in internal/redis/redis.go
4. Add the REDIS_USERNAME environment variable mapping

This is a valid feature request for supporting Redis instances that require ACL-based authentication.