Add get_broadcasts_stats method

[n7studios]

Summary

Adds a method for fetching a list of broadcast's stats.

Testing

• testGetBroadcastsStats : Test that get_broadcasts_stats() returns the expected data and pagination works.
• testGetBroadcastsStatsWithTotalCount: Test that get_broadcasts_stats() includes the total count when required.

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
• The code passes when running PHPStan
• 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)

[noelherrick]

I don't know how necessary it is to test pagination on every request that uses pagination. Thoughts?

[n7studios]

I don't know how necessary it is to test pagination on every request that uses pagination. Thoughts?

Probably overkill, but it would cover a scenario where (I!) accidentally were to remove pagination arguments from a method, as tests would then fail.