Is this a bug report or feature request?

• Feature Request

What should the feature do:
An admission controller would improve the feedback to admins creating CRs when there are error conditions. The static openapiv3 schema checking can only do so much to check for the errors. Without the admission controller, the operator has to check for invalid settings when applying the CR and errors would just be logged. For example, some error checks in the admission controller could include checking for the following invalid configurations:

CephCluster:

• Cannot change to or from host networking after the cluster is initially created
• Cannot change the dataDirHostPath after initial creation
• Check for resource limits that are too low

CephObjectPool/CephFilesystem/CephObjectStore:

• Both replication and erasure coding settings are specified

and many more scenarios...

What is use case behind this feature:
Better error validation.

Related to #2363, just specific to Ceph.