support more statistical metrics calculations, including refined code for calculating quantile numbers, calculating distinct value proportion value, null value proportion, empty value proportion, mean value, max/min value, stderr, std, max/min length for datatype and so forth. Besides, support for multiple data types.
update data summary #13

add UT for DataSummary ET
import byzer-extension-tests #16
Varied the data types when count null value for columns and add timestamp mock data into unit test #14