[Vertex AI] Add image generation support using Imagen · firebase/firebase-ios-sdk@bb7dab5

Skipped FirebaseVertexAIUnit.ChatSnippets.testChatNonStreaming

Skipped FirebaseVertexAIUnit.ChatSnippets.testChatStreaming

Skipped FirebaseVertexAIUnit.FunctionCallingSnippets.testFunctionCalling

Skipped FirebaseVertexAIUnit.MultimodalSnippets.testMultimodalMultiImagesNonStreaming

Skipped FirebaseVertexAIUnit.MultimodalSnippets.testMultimodalMultiImagesStreaming

Skipped FirebaseVertexAIUnit.MultimodalSnippets.testMultimodalOneImageNonStreaming

Skipped FirebaseVertexAIUnit.MultimodalSnippets.testMultimodalOneImageStreaming

Skipped FirebaseVertexAIUnit.MultimodalSnippets.testMultimodalVideoNonStreaming

Skipped FirebaseVertexAIUnit.MultimodalSnippets.testMultimodalVideoStreaming

Skipped FirebaseVertexAIUnit.StructuredOutputSnippets.testStructuredOutputEnumBasic

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