[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 https://github.com/databrickslabs/lsql/pull/210#discussion_r1673681141_