* filer: add username and keyPrefix support for Redis stores
Addresses https://github.com/seaweedfs/seaweedfs/issues/7299
- Add username config option to redis2, redis_cluster2, redis_lua, and
redis_lua_cluster stores (sentinel stores already had it)
- Add keyPrefix config option to all Redis stores to prefix all keys,
useful for Envoy Redis Proxy or multi-tenant Redis setups
* refactor: reduce duplication in redis.NewClient creation
Address code review feedback by defining redis.Options once and
conditionally setting TLSConfig instead of duplicating the entire
NewClient call.
* filer.toml: add username and keyPrefix to redis2.tmp example
