Speed up dynamodb List() by only getting keys (#21159)

* Speed up ddb list by only getting keys

* Add same optimization to has_children

* Add changeling

---------

Co-authored-by: Violet Hynes <violet.hynes@hashicorp.com>

changelog/21159.txt

@@ -0,0 +1,4 @@
+```release-note:improvement
+storage/dynamodb: Speed up list and delete of large directories by only requesting keys from DynamoDB
+```
+

physical/dynamodb/dynamodb.go

@@ -410,6 +410,11 @@ func (d *DynamoDBBackend) List(ctx context.Context, prefix string) ([]string, er
 				}},
 			},
 		},
+		ProjectionExpression: aws.String("#key, #path"),
+		ExpressionAttributeNames: map[string]*string{
+			"#key":  aws.String("Key"),
+			"#path": aws.String("Path"),
+		},
 	}
 
 	d.permitPool.Acquire()
@@ -453,6 +458,11 @@ func (d *DynamoDBBackend) hasChildren(prefix string, exclude []string) (bool, er
 				}},
 			},
 		},
+		ProjectionExpression: aws.String("#key, #path"),
+		ExpressionAttributeNames: map[string]*string{
+			"#key":  aws.String("Key"),
+			"#path": aws.String("Path"),
+		},
 		// Avoid fetching too many items from DynamoDB for performance reasons.
 		// We want to know if there are any children we don't expect to see.
 		// Answering that question requires fetching a minimum of one more item