feat: OSConfig: add OS policy assignment rpcs (#391)

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Committer: @adjackura
PiperOrigin-RevId: 407422484

Source-Link: googleapis/googleapis@c11bc3e

Source-Link: https://github.com/googleapis/googleapis-gen/commit/d2df80a5a61299ed2dff12bfaf2886619117e9ce
Copy-Tag: eyJwIjoiLmdpdGh1Yi8uT3dsQm90LnlhbWwiLCJoIjoiZDJkZjgwYTVhNjEyOTllZDJkZmYxMmJmYWYyODg2NjE5MTE3ZTljZSJ9