[Vertex AI] Add HarmSeverity enum and SafetyRating properties

[andrewheard]

Added the HarmSeverity enum and added the properties probabilityScore, severity, severityScore and blocked to SafetyRating.

[andrewheard]

cc: @rachelsaunders I linked to the Cloud docs since there isn't anything about severity at https://firebase.google.com/docs/vertex-ai/safety-settings but flagging in case you're planning to add it.

[rachelsaunders]

cc: @rachelsaunders I linked to the Cloud docs since there isn't anything about severity at https://firebase.google.com/docs/vertex-ai/safety-settings but flagging in case you're planning to add it.

Thanks for flagging this, Andrew! I've filed a ticket to update this in our narrative guide docs :-) When that's done, we can update the link here in the source code and ref docs.