feat: add Live Stream API channel clip samples and tests

[irataxy]

Description

Fixes b:341726237

Important: FFmpeg (https://www.ffmpeg.org/) must be installed in the test environment.

sudo apt install ffmpeg

These samples follow the existing format of the Live Stream API samples.

Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• media/livestream/createChannelClip.js:20, tag livestream_create_channel_clip
• media/livestream/deleteChannelClip.js:20, tag livestream_delete_channel_clip
• media/livestream/getChannelClip.js:20, tag livestream_get_channel_clip
• media/livestream/listChannelClips.js:20, tag livestream_list_channel_clips

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