feat: add samples and tests for adding captions to a job

[irataxy]

Needs https://cloud.google.com/transcoder/images/caption.srt uploaded to the test bucket for the tests to pass.

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Fixes 210892114

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/create_job_with_embedded_captions.py:24, tag transcoder_create_job_with_embedded_captions
• samples/snippets/create_job_with_standalone_captions.py:24, tag transcoder_create_job_with_standalone_captions

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[busunkim96]

Hey @irataxy, do you have an AI DPE helping you with getting that new file to the bucket or do you need someone else to look into that?

[irataxy]

Hey @irataxy, do you have an AI DPE helping you with getting that new file to the bucket or do you need someone else to look into that?

@nicain added it so this should be ready for review.