feat: create a function to filter problems

Create a function - 
if we give param "COOK", "LONG", "LUNCH" or "START" to that function
it should return problems to a particular contest.

E.g. - Given "COOK" and "LONG" to function - it should return Problems Model Object List containing problems to Long Challenge and Cook-Off. 

Steps - 
Filter Contests using some filter.
Filter Problems that are in that contests.