Validate command gracefully handles views

Currently, the validate command fails on a view namespace with an "ns not found" message. We should update the validate command to check for a view and validate its definitiion:

• The view has no unexpected fields
• The view does not refer to itself
• Each field in the view definition is the correct type
• The view has a valid aggregation pipeline

Alternatively, we can choose to not support validate on views. If this is the case, we should fail with ErrorCodes::CommandNotSupportedOnView.