Add Linear Regression using Gradient Descent

[abhinavprakash-x]

Description of Change

Added linear_regression.cpp which

• Implements Linear Regression with batch gradient descent
• Supports feature normalization and L2 regularization
• Includes a test with multiple features

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:
Implements Linear Regression, Z Score Normalization, and Gradient Descent.