Voice Additions, DTMF + NCCO

[SecondeJK]

This PR adds the synchronous and asynchronous modes to the Input NCCO, and the DTMF realtime event endpoints to the Voice Client.

Description

When using an Input DTMF, you can now set the mode. You cannot add DTMF settings to an Input NCCO if you have set it to async.

There are also two new endpoints, called with the following methods:
subscribeToDtmfEventsById() where you can add eventUrls to post DTMF events to
unsubscribeToDtmfEventsById() where you can stop the listener.

This gives overall control of whether you want individual events per DTMF keypress or whether to get the input keystrokes back batched.

Motivation and Context

Parity with voice API features.

How Has This Been Tested?

Tests added for this functionality.

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.