feat(Live Stream): add samples and tests for assets and pools

[irataxy]

b:264616218
Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 6 region tags.

• media/livestream/src/create_asset.php:27, tag livestream_create_asset
• media/livestream/src/delete_asset.php:27, tag livestream_delete_asset
• media/livestream/src/get_asset.php:27, tag livestream_get_asset
• media/livestream/src/get_pool.php:27, tag livestream_get_pool
• media/livestream/src/list_assets.php:27, tag livestream_list_assets
• media/livestream/src/update_pool.php:27, tag livestream_update_pool

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