docs: add code snippets for async api

[changsongd]

Previous PR is here: #5

The previous PR was deleted because @dizcology suggested that using the auto-generated code samples can already serve our purpose and can also provide the benefits of easy maintainability. However, after reviewing our product documents, TPM and eng raised the concern that customers may feel confused about the linked code samples. The code sample construct the request in the code. But in reality, customers tend to construct the request in a json file. Thus they may find it hard to parse and use the json file without us showing them how to do it in code. This PR is to add a code snippet for the sync API that reads the request from a json file.

[snippet-bot]

Here is the summary of possible violations 😱

There is a possible violation for not having product prefix.

• samples/snippets/async_api.py:15, tag cloudoptimization_async_api

The end of the violation section. All the stuff below is FYI purposes only.

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Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/async_api.py:15, tag cloudoptimization_async_api

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[changsongd]

Here is the summary of possible violations 😱

There is a possible violation for not having product prefix.
The end of the violation section. All the stuff below is FYI purposes only.

Here is the summary of changes.

You are about to add 1 region tag.
This comment is generated by snippet-bot. If you find problems with this result, please file an issue at: https://github.com/googleapis/repo-automation-bots/issues. To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment

Optimization AI is in restricted GA (GA with allowlist). The region tag prefix can be found here: https://devrel.corp.google.com/admin/products/463?t=snippets