mark default_from_api for empty_dir.medium in Cloud Run API.

[bskaplan]

empty_dir.medium for cloud run currently only supports a value of "memory". If it's unset, we use that as the default.

Like with other fields in Cloud Run, we plan to start explicitly capturing the default value and returning it on subsequent gets. Pre-emptively marking the default so Terraform users don't break when the server starts setting this field.

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• [x] Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
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