feat(v2/redis): add SentinelUsername and SentinelPassword to Config

Parent Issue: #110 (closed)

Note

This MR is a follow up action for !497 (merged), please retarget to master after it is merged.

Adds SentinelUsername and SentinelPassword fields to redis.Config and wires them through to goredis.UniversalOptions.

In security-hardened deployments, Sentinel processes authenticate independently from Redis data nodes. Previously, callers had no way to supply Sentinel-tier credentials through the labkit client.