Diameter of BST added

[marizvi]

Fixes #805

By submitting this pull request I confirm I've read and complied with the below declarations.

• I have read the Contribution guidelines and I am confident that my PR reflects them.
• I have followed the coding guidelines for this project.
• My code follows the skeleton code structure.
• This pull request has a descriptive title. For example, Added {Algorithm/DS name} [{Language}], not Update README.md or Added new code.
• This pull request will be closed if I fail to update it even once in a continuous time span of 7 days.
• This pull request shall only be reviewed and merged once the Travis build passes. No maintainer or supporter shall be obliged to review it before this condition is met.
• I have mentioned the issue number correctly (with hyperlink) in this pull request description.

Input Screenshot is attached below which is our part of code as well.

Output Screenshot :

If everything looks fine then kindly merge my PR as well, under contribute-a-thon'21.