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.

tests/median_pruner_tests.rs

@@ -13,6 +13,7 @@ fn trial_with_values(id: u64, intermediate_values: Vec<(u64, f64)>) -> Completed
         HashMap::new(),
         0.0,
         intermediate_values,
+        HashMap::new(),
     )
 }