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/text-from-text.rs

@@ -0,0 +1,26 @@
+use std::sync::Arc;
+
+use gemini_rs::prelude::*;
+
+use gcp_auth::AuthenticationManager;
+
+#[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 prompt = "What is the airspeed of an unladen swallow?";
+    let result = vertex_client.prompt_text(prompt, None).await?;
+    println!("Response: {}", result);
+
+    Ok(())
+}