Improvement: Support update operation for inspection policy

[JingChen23]

Several things need to consider:

1. Handle the modification of the workspace namespace.
2. Handle the modification of the cronjob frequency.
3. Handle the modification of scannners.
4. Handle the change of the policy name.

Currently we always need to destroy the policy then recreate it, to make the policy "update". This will cause the old cronjob be deleted immediately, not very grace.