Add validation and throw proper error when Cluster resource without ClusterClass is passed with cluster create command

[anujc25]

Bug description
Currently, when user passes Cluster resource as part of tanzu cluster create --file <input.yaml> and the cluster resource doesn't include ClusterClass as part of .spec.topology.cluster a wrong error message is thrown which can be misleading to the user.

As part of the error message, throw meaningful error message as this usecase is not yet supported.

Affected product area (please put an X in all that apply)

• ( ) APIs
• ( ) Addons
• ( ) CLI
• ( ) Docs
• ( ) IAM
• ( ) Installation
• ( ) Plugin
• ( ) Security
• ( ) Test and Release
• ( ) User Experience
• ( ) Developer Experience

Expected behavior
Should throw meaningful error message as this usecase is not yet supported.

Steps to reproduce the bug

Version (include the SHA if the version is not obvious)

Environment where the bug was observed (cloud, OS, etc)

Relevant Debug Output (Logs, manifests, etc)