DEVX-6803: Adding Captions API

[superchilled]

Description

This PR does the following:

Implements the Captions API functionality. Specifically:

• Implements a Caption class with start and stop wrapper functions
• Updates the Client class to add start_live_captionsandstop_live_captionswhich are called by theCaptions` wrappers
• Updates the Opentok class to add a captions method which returns a Captions object
• Adds a OpenTokCaptionsError class
• Adds unit tests for the new functionality

Motivation and Context

Adds a feature to the SDK which has recently gone GA in the API.

How Has This Been Tested?

Unit tests added for the new functionality.

Example Output or Screenshots (if appropriate):

n/a

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.

[superchilled]

Hi @jeffswartz the Ruby SDK reference docs will need to be updated for this release. :)

[maxkahan]

A thing of beauty ✨