ChatAnthropicVertex prompt caching support

[jthack]

Hello,

As of recently, prompt caching is supposedly in preview in Vertex AI. Can you add support for it to ChatAnthropicVertex?

Thanks!

[ShaharZivanOnvego]

I just want to bump this, and clarify that the "regular" method I use for cache prompting in the standard ChatAnthropic causes an error:

        content = [{
            "text": "Do something or other...",
            "type": "text",
            "cache_control": {"type": "ephemeral"}
        }]   
        
        prompt = ChatPromptTemplate.from_messages(
            [
                SystemMessage(content=content),
                ("placeholder", "{messages}"),
            ]
        )

This method fails when giving this prompt to ChatAnthropicVertex with the error:

File ".../python3.11/site-packages/langchain_google_vertexai/_anthropic_utils.py", line 143, in _format_messages_anthropic raise ValueError( ValueError: System message must be a string, instead was: <class 'list'>

So simply modifying it to support a list rather than a string would be enough to allow caching. Could be a quick fix