[Vertex AI] Add `APIConfig` to support switching to the Developer API · firebase/firebase-ios-sdk@968bd5b

Skipped FirebaseVertexAIUnit.ChatSnippets.testChatNonStreaming

Skipped FirebaseVertexAIUnit.ChatSnippets.testChatStreaming

Skipped FirebaseVertexAIUnit.FunctionCallingSnippets.testFunctionCalling

Skipped FirebaseVertexAIUnit.MultimodalSnippets.testMultimodalVideoNonStreaming

Skipped FirebaseVertexAIUnit.MultimodalSnippets.testMultimodalVideoStreaming

Skipped FirebaseVertexAIUnit.StructuredOutputSnippets.testStructuredOutputEnumBasic

Skipped FirebaseVertexAIUnit.StructuredOutputSnippets.testStructuredOutputJSONBasic

Skipped FirebaseVertexAIUnit.TextSnippets.testTextOnlyNonStreaming

Skipped FirebaseVertexAIUnit.TextSnippets.testTextOnlyStreaming

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