Make MemoryDB ClusterName Optional

### Name of the resource

AWS::MemoryDB::Cluster

### Resource name

_No response_

### Description

When creating a MemoryDB cluster, the ClusterName property is required. MemoryDB could be a used to replace an existing ElastiCache Redis Cluster. `AWS::ElastiCache::CacheCluster` does not require `ClusterName` be provided and CloudFormation will generate a unique ID for the ElastiCache cluster name. MemoryDB for Redis should follow this same design.

### Other Details

_No response_

The issue concerns the AWS::MemoryDB::Cluster resource and compares it with AWS::ElastiCache::CacheCluster; start by reviewing both resource specifications and how ClusterName is currently required. Done means MemoryDB permits CloudFormation to generate a unique cluster name when ClusterName is omitted, with equivalent validation or coverage documented.

Written by the indexing model from the issue text.