Feat/ New DS input for file peeking

[ArturU043]

Adding new input types to get_structure and related test
Users can now input ServiceX DataSetIdentifiers like:

from servicex import dataset 
from servicex_analysis_utils import get_structure
ds1 = dataset.Rucio("space:did")
ds2 = dataset.CERNOpenData("cernopendata:12345")
dict= {"private" : ds1, "public": ds2}

get_strucre(dict)

This solves #18 #9