[BUG FIX] fix a bug for error when using openai function agent & add some examples

[diemus]

PR Checklist

• [x] Read the Contributing documentation.
• [x] Read the Code of conduct documentation.
• [x] Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• [x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• [x] Describes the source of new concepts.
• [x] References existing implementations as appropriate.
• [x] Contains test coverage for new functions.
• [x] Passes all golangci-lint checks.

Currently, the OpenAI function agent has a bug. When executing a tool, it throws an error: API returned unexpected status code: 400: Missing parameter 'name': messages with role 'function' must have a 'name'. The reason is that while processing the result of the previous action, the message type is FunctionChatMessage instead of ToolChatMessage, and the Plan function does not handle FunctionChatMessage. Therefore, the function result turns into a regular user message, causing OpenAI to return a 400 error.

The issue is very easy to reproduce:

package main

import (
    "context"
    "fmt"
    "github.com/tmc/langchaingo/callbacks"
    "log"
    "os"

    "github.com/tmc/langchaingo/agents"
    "github.com/tmc/langchaingo/llms/openai"
    "github.com/tmc/langchaingo/tools"
)

func main() {
    if err := run(); err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }
}

func run() error {
    llm, err := openai.New(openai.WithModel("gpt-4-turbo"))
    if err != nil {
        log.Fatal(err)
    }

    agentTools := []tools.Tool{
        tools.Calculator{},
    }

    agent := agents.NewOpenAIFunctionsAgent(
        llm,
        agentTools,
        agents.WithCallbacksHandler(callbacks.LogHandler{}),
    )
    if err != nil {
        return err
    }

    executor := agents.NewExecutor(agent,
        agents.WithMaxIterations(3),
        agents.WithReturnIntermediateSteps(),
    )

    a, err := executor.Call(context.Background(), map[string]any{"input": "what is 3 plus 3 and what is python"})
    fmt.Println(a, err)
    return err
}

I have already submitted a fix and added the example to the repository.