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.

.gitignore

@@ -1,14 +1,7 @@
-# Generated by Cargo
-# will have compiled files and executables
 debug/
 target/
-
-# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
-# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
 Cargo.lock
-
-# These are backup files generated by rustfmt
 **/*.rs.bk
-
-# MSVC Windows builds of rustc generate these, which store debugging information
 *.pdb
+/target
+.cargo/config.toml