support stream output

Changes in Cargo.toml:

 • Updated tokio from 1.0 to 1.0-beta.1.
 • Added google-generative-ai-rs as a dependency with version 0.1.7.

Changes in src/main.rs:

 • Added the shellexpand crate.
 • Made config_path configurable via command-line argument --config-file.
 • Added a --stream command-line argument to indicate whether the response should be streamed.

Changes in gemini-cli-example.toml:

 • Added an example config with google-generative-ai-rs model specified.
 • Set template to Context: {system}\nMessages:\nAuthor: User\nContent: {prompt}.

Commit message: Update Tokio, add support for Google Generative AI RS, and configurable streaming