Chain does not support images

[samuelint]

Describe the bug
Using chain (ConversationalChainBuilder or Agent) does not support images.

prompt_args! {
            "input" => input,
        }

does not allow to pass an image url.
If the url is passed as a string the in input, an error is thrown OpenAI error: stream failed: Invalid status code: 400 Bad Request probably because the image is not under the right structure (Image content part).

Also
langchain_rust::schemas::Message::messages_to_string only serialize text content. It does not take in consideration images.

langchain-rust/src/schemas/messages.rs

Lines 149 to 155 in 891c56f

	pub fn messages_to_string(messages: &[Message]) -> String {
	messages
	.iter()
	.map(|m| format!("{:?}: {}", m.message_type, m.content))
	.collect::<Vec<String>>()
	.join("\n")
	}

To Reproduce

let chain = ConversationalChainBuilder::new()
            .llm(llm)
            .build();

let base64_image_url = "data:image/jpeg;base64,23434234234234".to_string();
let input = format!("What is that? {}", base64_image_url);
let result = chain.invoke(prompt_args! {
    "input" => input,
});

Expected behavior
A way to pass image to a chain or at minimum an Agent.

Since images are on langchain_rust::schemas::Message structure, I would expect them to work in every aspect of the library.

Additional context
This is some kind of a violation of the Liskov Principle & Interface Segregation Principles. If image are available on langchain_rust::schemas::Message they should be supported everywhere in the library.