feat(samples): add samples and tests for adding captions to videos

[irataxy]

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Fixes b:210892114

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/createJobWithEmbeddedCaptions.js:19, tag transcoder_create_job_with_embedded_captions
• samples/createJobWithStandaloneCaptions.js:19, tag transcoder_create_job_with_standalone_captions

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[bcoe]

LGTM. My only concern with tests that create resources (which I'm sure I've mentioned before 😆) is what happens if a test fails, will we gradually leak the resources?

Could we add an after step that deletes jobs older than N days? Do we do this elsewhere?