Solution of task - Group people by city

[zulqer-nain]

• Added solution to group people by city.
• Added solution to group people by gender.
• Added generic function "groupBy" to group by any property .
• Added test case for group people by city
• Added test case for group people by gender.

[krajasekhar-unify]

Also please link the original issue for this pull request in the Linked Issues section.