DEVX-5655 Implement dial DTMF digits feature

[superchilled]

Description

Implements the play DTMF feature, either for an individual connection or for all connections in a session. Two methods have been added, one for each of these actions, as well as a helper method for validating DTMF digits.

Motivation and Context

It implements a feature in the SDK which has already been added to the API.

How Has This Been Tested?

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.