`CredentialsProvider` is missing from `ClusterOptions`

The `ClusterOptions` struct is missing a [`CredentialsProvider`](https://github.com/redis/go-redis/blob/21bd40a47e56e61c0598ea1bdf8e02e67d1aa651/options.go#L62) field for symetry.

As a workaround, I can use the [`NewClient`](https://github.com/redis/go-redis/blob/21bd40a47e56e61c0598ea1bdf8e02e67d1aa651/osscluster.go#L36) field to inject my provider when a client is created.