Enhanced Image & Multi-Part Message Handling

[LuticaCANARD]

What Is added

• for processing base64-encoded Image, I added use-base64 feature, and added base64 dependency as optional dependency.

you can try via
cargo run --features use-base64 --package gemini-rs --example generate_image

• To enhance context provision for users, I have introduced the send_parted_messages function, which allows for sending multiple parts in a single request.
• Added a convenience method for creating Part::inline_data, simplifying the API and improving developer ergonomics.