Added ability to list broadcasts

[dragonmantank]

Description

Fixes #241

This adds the ability to list broadcasts, which was overlooked at some point.

Motivation and Context

This is part of the REST API and should be covered.

How Has This Been Tested?

Unit tests

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.

[codecov]

Codecov Report

Merging #288 (b589418) into main (1cdbe85) will increase coverage by 0.3%.
The diff coverage is 83.3%.

❗ Current head b589418 differs from pull request most recent head ad2fc71. Consider uploading reports for the commit ad2fc71 to get more accurate results
| Impacted Files | Coverage Δ | |
|---|---|---|
| src/OpenTok/Util/Validators.php | 69.0% <77.7%> (+0.4%) | ⬆️ |
| src/OpenTok/BroadcastList.php | 81.8% <81.8%> (ø) | |
| src/OpenTok/Util/Client.php | 73.9% <82.3%> (+0.3%) | ⬆️ |
| src/OpenTok/OpenTok.php | 92.9% <100.0%> (+0.2%) | ⬆️ |