provider/aws: Add reader_endpoint RDS Clusters (supersedes #8878) (#8884)
provider/aws: Add reader_endpoint RDS Clusters (supersedes #8878) (#8884)

* provider/aws: Add support for Aurora ReaderEndpoint

* provider/aws: Add reader_endpoint to the schema, document, test

charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,terraform-providers/terraform-provider-aws