Add sense field to Instance (#28)

# 変更点

OMMX::Instanceが意味する最適化問題が最小化問題であるか最大化問題であるかが判別できないという課題があった為、senseを追加しました。

# 確認事項

こちらの変更後、Pythonパッケージをv0.1.2として、Rustクレートをv0.1.1としてリリースしても良いでしょうか？（バージョニングは適切でしょうか？）

---------

Co-authored-by: Toshiki Teramura <toshiki.teramura@gmail.com>

proto/ommx/v1/instance.proto

@@ -31,4 +31,18 @@ message Instance {
   //
   // TODO: Add semi-definite constraints to represent SDP
   // repeated SemiDefiniteConstraint semi_definite_constraints = ?;
+
+  // The sense of this instance
+  enum Sense {
+    SENSE_UNSPECIFIED = 0;
+    SENSE_MINIMIZE = 1;
+    SENSE_MAXIMIZE = 2;
+  }
+
+  // The sense of this problem, i.e. minimize the objective or maximize it.
+  //
+  // Design decision note:
+  // - This is a required field. Most mathematical modeling tools allow for an empty sense and default to minimization. Alternatively, some tools do not create such a field and represent maximization problems by negating the objective function. This project prefers explicit descriptions over implicit ones to avoid such ambiguity and to make it unnecessary for developers to look up the reference for the treatment of omitted cases.
+  //
+  Sense sense = 5;
 }

python/pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "ommx"
-version = "0.1.1"
+version = "0.2.0"
 description = "Open Mathematical prograMming eXchange (OMMX)"
 authors = [
     { name="Jij Inc.", email="info@j-ij.com" },

rust/ommx/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ommx"
-version = "0.1.0"
+version = "0.2.0"
 edition = "2021"
 license = "MIT OR Apache-2.0"