resource/aws_db_cluster_snapshot: Add plan-time validation for db_clu…

…ster_snapshot_identifier argument (#15132)

Output from acceptance testing:

```
--- PASS: TestAccAWSDBClusterSnapshot_basic (177.55s)
--- PASS: TestAccAWSDBClusterSnapshot_Tags (194.42s)
```

aws/resource_aws_db_cluster_snapshot.go

@@ -3,12 +3,14 @@ package aws
 import (
 	"fmt"
 	"log"
+	"regexp"
 	"time"
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/service/rds"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
+	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/validation"
 	"github.com/terraform-providers/terraform-provider-aws/aws/internal/keyvaluetags"
 )
 
@@ -33,6 +35,13 @@ func resourceAwsDbClusterSnapshot() *schema.Resource {
 				Type:     schema.TypeString,
 				Required: true,
 				ForceNew: true,
+				ValidateFunc: validation.All(
+					validation.StringLenBetween(1, 63),
+					validation.StringMatch(regexp.MustCompile(`^[0-9a-z-]+$`), "must contain only lowercase alphanumeric characters and hyphens"),
+					validation.StringMatch(regexp.MustCompile(`^[a-z]`), "must begin with a lowercase letter"),
+					validation.StringDoesNotMatch(regexp.MustCompile(`--`), "cannot contain two consecutive hyphens"),
+					validation.StringDoesNotMatch(regexp.MustCompile(`-$`), "cannot end with a hyphen"),
+				),
 			},
 			"db_cluster_identifier": {
 				Type:     schema.TypeString,