CaS predicate bounds

Current implementation of cas::Predicate implies inclusive ranges and doesn't let to specify excluded or unbounded ranges.

Let's refactor the API according to the proposal: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e64790625d5f5627f56b50c103126241

diff --git a/src/traft/rpc/cas.rs b/src/traft/rpc/cas.rs
index f077bdd9..47f0952e 100644
--- a/src/traft/rpc/cas.rs
+++ b/src/traft/rpc/cas.rs
@@ -291,10 +291,15 @@ pub struct Predicate {
 pub struct Range {
     pub space: String,
     pub index: String,
-    #[serde(with = "serde_bytes")]
-    pub key_min: TupleBuffer,
-    #[serde(with = "serde_bytes")]
-    pub key_max: TupleBuffer,
+    pub key_min: Bound<TupleBuffer>,
+    pub key_max: Bound<TupleBuffer>,
 }

+#[derive(Clone, Debug, serde::Serialize, serde::Deserialize)]
+pub enum Bound<T> {
+    Included(T),
+    Excluded(T),
+    Unbounded,
+}