Added Binary_Exponentiation in Java

[anshkush92]

Have you read the Contribution Guidelines?

Yes

Description

Added binary exponentiation in java with comments explaining them Added Testcase on which I checked the program Added Time and Space Complexity

Checklist

• [x] I've read the contribution guidelines.
• [x] I've checked the issue list before deciding what to submit.
• [x] I've edited the README.md and link to my code.

Related Issues (Mention Issue no. like #xxx)

#212 Fixes Issue
(Write your answer above.)