Add OS_INVENTORY to CloudAsset feed contentTypes

[modular-magician]

This adds OS_INVENTORY to contentType for cloud asset feeds like google_cloud_asset_project_feed.
The option is specified here: https://cloud.google.com/asset-inventory/docs/overview#content_types but not present in the current provider.

Note: as this is a very small change -- I did not open an issue (i did search though!) or run the acceptance tests.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• 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).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

cloudasset: added `OS_INVENTORY` value to `content_type` for `google_cloud_asset_*_feed`

Derived from GoogleCloudPlatform/magic-modules#7672