Add support for prompt variables (bot name, date/time, model id)

Fixes https://github.com/etkecc/baibot/issues/10

This also includes them in the default prompts (for newly-created agents),
so that people can get a better experience out of the box.

src/agent/provider/entity/mod.rs

@@ -9,5 +9,7 @@ pub use agent_provider::{AgentProvider, AgentProviderInfo};
 pub use image_generation::{ImageGenerationParams, ImageGenerationResult};
 pub use ping::PingResult;
 pub use speech_to_text::{SpeechToTextParams, SpeechToTextResult};
-pub use text_generation::{TextGenerationParams, TextGenerationResult};
+pub use text_generation::{
+    TextGenerationParams, TextGenerationPromptVariables, TextGenerationResult,
+};
 pub use text_to_speech::{TextToSpeechParams, TextToSpeechResult};