fix(firebase_vertexai): add missing HarmBlockThreshold to exported APIs

[rusudinu]

Description

The HarmBlockThreshold added in version 1.5.0 of firebase_vertexai is missing from the exported APIs. This PR adds it to the list.

This PR fixes the following scenario in which, I want to use HarmBlockMethod.probability like this:
SafetySetting(HarmCategory.hateSpeech, HarmBlockThreshold.high, HarmBlockMethod.probability),
and being unable to only with the following import:
import 'package:firebase_vertexai/firebase_vertexai.dart'; and I would need to also add
import 'package:firebase_vertexai/src/api.dart'; which would be an anti-pattern.

TLDR: HarmCategory and HarmBlockThreshold were already present, HarmBlockMethod was missing and this PR adds it to the exports list.

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