Add auth_token_update_strategy option

[karlismelderis]

Currently, any auth_token changes are destroying and are recreating Redis cluster.

AWS API has option to set new token and chose update strategy: SET or ROTATE

https://docs.aws.amazon.com/AmazonElastiCache/latest/APIReference/API_ModifyCacheCluster.html

[Routhinator]

This is a pretty key feature, FWIW. Security token cycling is something that should be able to be done frequently and with minimal disruption.... replacing the cluster adds a lot of overhead.

[Nuru]

This is blocked by hashicorp/terraform-provider-aws#11524

[Steve-Louie-Bose]

Looks like it's supported now. hashicorp/terraform-provider-aws#16203