[NEP-19833]: Fetching normal and shared datasets separately

[soundarya-mv]

1. Dashboard::Datasets::List.new now accepts a new parameter separate_shared_datasets which when true makes the instance method dataset_details to return the results this way
   { 'datasets' => [], 'shared_datasets' => [] }
   Here shared_datasets holds the datasets that are being shared across different dashboards filters.
   Eg: On the books month dataset
   This is useful when we do a bulk delete cascade for recreation of dashboards to avoid the deletion of shared datasets which will cause all the dashboards using the dataset to break.

To set the dataset as shared dataset, we use the extra field in superset dataset settings.

2. DuplicateDashboard class accepts a new parameter ignore_duplicate_datasets which when true, does not throw validation error if a dataset is already present in tenant during dashboard duplication and proceeds with duplication.