improve validation process

current json and pydantic validation only raises exceptions. we may also want to consider levels of validation, e.g., `ALL`, `WARN`, `CRITICAL`

`ALL`: "would be nice if all these metadata were provided"
`WARN`: "we don't know if this is applicable to you, but it was not provided"
`CRITICAL`: "missing metadata - dandiset cannot be published without these"

level names are suggestions at this point.

once this is implemented:
1. the api server would need to adjust how it returns validation info, and that in turn impacts the gui.
2. the cli would also need to interpret these levels appropriately.

this would also be tied to automigration of any draft dandiset/asset metadata. the validator should be able to indicate to the user if automigration would solve any validation issues or whether human intervention (reupload, reconvert, edit metadata in gui, etc.) is required.