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/mod.rs

@@ -21,5 +21,5 @@ pub use config::ConfigTrait;
 
 pub use entity::{
     AgentProvider, AgentProviderInfo, ImageGenerationParams, PingResult, SpeechToTextParams,
-    SpeechToTextResult, TextGenerationParams, TextToSpeechParams,
+    SpeechToTextResult, TextGenerationParams, TextGenerationPromptVariables, TextToSpeechParams,
 };