feat(elasticache-redis): add engine input for valkey support

[nitrocode]

what

• add engine input for valkey support

why

• Valkey is far cheaper than redis

Notes

• Design options
  1. Add a new key to local.cluster_attributes and add a variable to the component, default it to redis, and pass as an argument to the module as-is
     • We could shy away from the pattern to using engine = var.engine in the local.
  2. Allow var.redis_clusters to supply engine with a default for redis and pass to module as-is
  3. Same as option 2 but allow local.cluster_attributes to overwrite it
     • This might be best of both worlds however no other argument does this so it would be breaking the pattern
• Went with option 2 so the argument isn't a new requirement for everyone and it doesn't break consistency

references

• https://github.com/cloudposse/terraform-aws-elasticache-redis/releases/tag/v1.7.0
• https://aws.amazon.com/blogs/opensource/why-aws-supports-valkey/
• https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/elasticache_replication_group#engine
• [Enhancement]: Add Valkey support to Elasticache resources hashicorp/terraform-provider-aws#39641
• https://github.com/hashicorp/terraform-provider-aws/releases/tag/v5.73.0
• https://docs.aws.amazon.com/AmazonElastiCache/latest/dg/Clusters.Create.html
• https://www.lastweekinaws.com/blog/aws-valkey-play-when-a-fork-becomes-a-price-cut/

[GabisCampana]

@Nuru

[Nuru]

Please enumerate the legal values of engine in the variable description. Please also provide, in the README, an explanation of what they mean, with one or more links to even more information.

[nitrocode]

Please enumerate the legal values of engine in the variable description. Please also provide, in the README, an explanation of what they mean, with one or more links to even more information.

Thanks for reviewing. I'll add this now.

[nitrocode]

@Nuru changes addressed, please re-review.