Add base code and examples for the Vertex API

- Add base code for the Vertex API with types and a client with helper
  methods for prompting text and conversation.
- Add examples for prompting text and conversation.

examples/conversation.rs

@@ -0,0 +1,61 @@
+use std::{sync::Arc, time::Duration};
+
+use console::style;
+use dialoguer::{theme::ColorfulTheme, Input};
+use gemini_rs::prelude::*;
+
+use gcp_auth::AuthenticationManager;
+use indicatif::{ProgressBar, ProgressStyle};
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let authentication_manager = Arc::new(AuthenticationManager::new().await?);
+    let api_endpoint = std::env::var("API_ENDPOINT")?;
+    let project_id = std::env::var("PROJECT_ID")?;
+    let location_id = std::env::var("LOCATION_ID")?;
+
+    let vertex_client = VertexClient::new(
+        authentication_manager,
+        api_endpoint,
+        project_id,
+        location_id,
+    );
+
+    let mut conversation = Conversation::new();
+    loop {
+        let message: String = Input::with_theme(&ColorfulTheme::default())
+            .with_prompt("user")
+            .interact_text()?;
+
+        // Exit the conversation if the user types "exit"
+        if message == "exit" {
+            break;
+        }
+
+        // Push the user's message to the conversation.
+        conversation.push_message(Message::new(Role::User, &message));
+
+        // Show a spinner while the model is thinking.
+        let progress = ProgressBar::new_spinner();
+        progress.enable_steady_tick(Duration::from_millis(120));
+        progress.set_style(ProgressStyle::with_template("{spinner:.green} {msg}")?);
+        progress.set_message("Thinking...");
+
+        // Prompt the model with the conversation so far.
+        let response = vertex_client.prompt_conversation(&conversation).await?;
+
+        // Stop the spinner and clear the terminal.
+        progress.finish_and_clear();
+
+        // Print the model's response.
+        println!(
+            "✨ {} {} {}",
+            style(response.role.to_string()).bold(),
+            style("·").dim(),
+            style(&response.text).cyan()
+        );
+        conversation.push_message(response);
+    }
+
+    Ok(())
+}