[feature] Support CredentialsProvider for Redis/Valkey engine (IAM auth)

When using managed Valkey (Google Memorystore) with IAM-only authentication, there's no way to configure dynamic/short-lived credentials. Go-redis support CredentialsProvider for automatic token refresh, but Centrifugo doesn't expose this.

Use case: Google Memorystore for Valkey with --auth-mode=iam-auth — no static password, only IAM tokens (1h TTL).

Proposed: add support for a credentials provider callback or a configurable external command to retrieve Redis password dynamically.