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/lib.rs

@@ -0,0 +1,12 @@
+mod conversation;
+pub mod error;
+mod token_provider;
+mod types;
+mod vertex_client;
+
+pub mod prelude {
+    pub use crate::conversation::*;
+    pub use crate::token_provider::*;
+    pub use crate::types::*;
+    pub use crate::vertex_client::*;
+}