feat(vertexai): Add Live streaming feature

[cynthiajoan]

Description

Added preliminary support for bidirectional streaming. This feature is not yet fully supported.

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[russellwheatley]

The analyse CI run has some issues that need addressing: https://github.com/firebase/flutterfire/actions/runs/14028146820/job/39270346868?pr=16991#step:5:336

This test also failed for vertexAI: https://github.com/firebase/flutterfire/actions/runs/14028146820/job/39270348108?pr=16991#step:5:1341