[Vertex AI] Add HarmBlockMethod enum and method property

[andrewheard]

Added the HarmBlockMethod enum and added the method property to SafetySetting. Verified encoding when both methods are provided, as well as when method is omitted, in the integration test.

Aligned with Android / JS on method being optional with no default value (firebase/firebase-android-sdk#6379 and firebase/firebase-js-sdk#8567).

[andrewheard]

@rlazo Would you be opposed to changing internal val method: HarmBlockMethod = HarmBlockMethod.PROBABILITY to internal val method: HarmBlockMethod? = null ?

SafetySetting.method is specifically documented as "Optional" in the Vertex AI REST API docs and annotated as (google.api.field_behavior) = OPTIONAL in the proto (the category and threshold are (google.api.field_behavior) = REQUIRED).

@hsubox76 It looks like method is required in the JavaScript SDK -- though not strictly enforced by the TypeScript compiler IIUC.

cc: @cynthiajoan Since I think the Dart implementation is still pending.

On the server side, the Python SDK defaults this to None.

[andrewheard]

cc: @rachelsaunders I linked to the Cloud docs since there isn't anything about HarmBlockMethod in https://firebase.google.com/docs/vertex-ai/safety-settings.

[rachelsaunders]

https://firebase.google.com/docs/vertex-ai/safety-settings

Thanks for this additional flag. I think this will fall within the scope of the same ticket that I filed in response to the comment thread in the other PR.