Broadcasts: REST API: Improvements + Tests

[n7studios]

Summary

Improves #1013 by

• Changing the REST API route name to a more descriptive broadcasts,
• Changing the REST API verb from POST to GET
• Check if Broadcasts exist before attempting to return them
• Adds integration tests

Testing

• RESTAPIBroadcastsTest: Adds REST API tests for the broadcasts endpoint

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

[github-actions]

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