feat: add trial user attributes for logging and analysis

Add AttrValue enum (Float, Int, String, Bool) with From impls and
set_user_attr/user_attr/user_attrs methods on both Trial and
CompletedTrial. Attributes set during optimization are propagated
through complete_trial and prune_trial. AttrValue is re-exported
at crate root and in the prelude.

src/pruner/hyperband.rs

@@ -278,6 +278,7 @@ mod tests {
             HashMap::new(),
             0.0,
             values.to_vec(),
+            HashMap::new(),
         )
     }
 

src/pruner/percentile.rs

@@ -204,6 +204,7 @@ mod tests {
             HashMap::new(),
             0.0,
             values.to_vec(),
+            HashMap::new(),
         )
     }
 

src/pruner/successive_halving.rs

@@ -240,6 +240,7 @@ mod tests {
             HashMap::new(),
             0.0,
             values.to_vec(),
+            HashMap::new(),
         )
     }
 

src/pruner/wilcoxon.rs

@@ -311,6 +311,7 @@ mod tests {
             HashMap::new(),
             value,
             intermediate_values,
+            HashMap::new(),
         )
     }