[Go] Add persistent chat session and agent support

[apascal07]

https://firebase.google.com/docs/genkit/chat
https://firebase.google.com/docs/genkit/multi-agent

g, err := genkit.New()
if err != nil {
    log.Fatal(err)
}

updateTool := genkit.DefineTool(
    g, 
    "updateInfo", 
    "use this to set the party information",
    func(ctx context.Context, newName, partyTheme string) (string, error) {
        genkit.SessionFromContext(ctx, g).UpdateState(PartyDetails{ Name: newName, PartyTheme: partyTheme })
        return "changed name to " + newName + " and party theme to " + partyTheme, nil
    }
)
themeIdeasTool := g.Prompt("themeIdeasPrompt").AsTool()

type PartyDetails struct {
    Name string
    PartyTheme string
}

s, err := genkit.NewSession(
    g,
    chat.WithId("my-session"), // optional, defaults to uuid
    chat.WithStore(...), // optional, defaults to in-memory
    chat.WithStateType(PartyDetails{ Name: "Alex", PartyTheme: "Halloween" }), // pulls out type and initial values
)
if err != nil {
    log.Fatal(err)
}

// Mostly same options as `Generate` but redefined in their own package since not everything will apply and there are chat-specific ones.
c, err := genkit.NewChat(
    g,
    chat.WithSession(s), // optional, if left off then no state is kept
    chat.WithSystemText("You are a helpful assistant helping the customer create the best party ever."), 
    chat.WithTools(updateTool, themeIdeasTool),
)
if err != nil {
    log.Fatal(err)
}

res, err := c.Send("my name is Tom and I like animals") // optional chat.WithStreaming(...)
if err != nil {
    log.Fatal(err)
}

fmt.Println(res)