Added ability to add a fallback URL on application

[SecondeJK]

Currently the application entity does not allow you a create a fallback answer URL. This adds the constant to allow that ability.

Description

Application webhooks now have a constant added to them to allow to create this type of webhook when creating or updating an application. Here is an example:

        $application = new Application();
        $application->setName('my application');
        $webhook = new Webhook('https://example.com/fallbackUrl', 'GET');
        $application->getVoiceConfig()->setWebhook(\Vonage\Application\VoiceConfig::FALLBACK_ANSWER_URL, $webhook);

Motivation and Context

Reported in #289

How Has This Been Tested?

Added a test to check the output

Example Output or Screenshots (if appropriate):

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.