Messages API to number validation

[SecondeJK]

This PR adds new validation for outgoing SMS messages.

Description

There are two new parts of the validation for the Messages API client:

• Developers can now add a leading + if they want to. The SDK will automatically remove it for you before sending the request to the API
• Once the plus has been removed, a new method checks to see if it is in valid E164 format. If not, it will throw an exception before it gets to the API.

Motivation and Context

The response from the API currently is a little bit confusing when a bad number has been sent. Rather than have the developer try and understand the response, it handles the number more gracefully in the SDK before the request is sent out.

How Has This Been Tested?

Tests have been included in the existing suite to check the result.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.