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/text_generation/mod.rs

@@ -0,0 +1,15 @@
+mod prompt_variables;
+
+pub use prompt_variables::TextGenerationPromptVariables;
+
+#[derive(Default)]
+pub struct TextGenerationParams {
+    pub context_management_enabled: bool,
+    pub prompt_override: Option<String>,
+    pub temperature_override: Option<f32>,
+    pub prompt_variables: TextGenerationPromptVariables,
+}
+
+pub struct TextGenerationResult {
+    pub text: String,
+}