feat: [LiveStream] add Channel event samples and tests

[irataxy]

b:241866780

Part 3 of 3: Channel events (see https://cloud.google.com/livestream/docs/how-to/create-channel-events)

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• media/livestream/src/create_channel_event.php:27, tag livestream_create_channel_event
• media/livestream/src/delete_channel_event.php:27, tag livestream_delete_channel_event
• media/livestream/src/get_channel_event.php:27, tag livestream_get_channel_event
• media/livestream/src/list_channel_events.php:27, tag livestream_list_channel_events

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