[TECH DEBT] Move create_dashboards to DashboardMetadata.as_lakeview

[JCZuurmond]

It looks like the entire create_dashboard method from here should be converted to a method on DashboardMetadata:

def as_lakeview(self) -> Dashboard:
  self.validate()
  datasets = self.get_datasets() # make get_datasets private
  layouts = self.get_layouts() # make get_layouts private
  page = Page(
    name=dashboard_metadata.display_name,
    display_name=dashboard_metadata.display_name,
    layout=layouts,
  )
  return Dashboard(datasets=datasets, pages=[page])

Originally posted by @nfx in #210 (comment)