Add Webhook Endpoint Validation

[peanutenthusiast]

Description

This PR implements

• validation for Twilio Messages, generic OpenedX event notifications
• unit tests for custom function validatePhoneNumber to validate US phone numbers starting with +1
• unit tests for custom function validateSafeString to validate against potential unsafe strings for SQL injection, XSS attacks, etc.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor
• Other (please describe):

How Has This Been Tested?

Added unit tests to check for the above custom functions

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code where necessary
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the documentation (if applicable)

Related Issues

Fixes #11 (if applicable)