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.

src/token_provider.rs

@@ -0,0 +1,28 @@
+use std::sync::Arc;
+
+use gcp_auth::AuthenticationManager;
+
+use crate::error::Result;
+
+pub trait TokenProvider {
+    fn get_token(&self, scope: &[&str])
+        -> impl std::future::Future<Output = Result<String>> + Send;
+}
+
+impl TokenProvider for Arc<AuthenticationManager> {
+    async fn get_token(&self, scope: &[&str]) -> Result<String> {
+        match AuthenticationManager::get_token(self, scope).await {
+            Ok(token) => Ok(token.as_str().to_string()),
+            Err(e) => Err(e.into()),
+        }
+    }
+}
+
+impl TokenProvider for AuthenticationManager {
+    async fn get_token(&self, scope: &[&str]) -> Result<String> {
+        match AuthenticationManager::get_token(self, scope).await {
+            Ok(token) => Ok(token.as_str().to_string()),
+            Err(e) => Err(e.into()),
+        }
+    }
+}