GH-46380: [GLib] Add GArrowFixedShapeDataType#shape (#46381)

### Rationale for this change

The C++ API implemented `FixedShapeTensor::shape()` instance method. GLib was not yet supported.

### What changes are included in this PR?

Add `GArrowFixedShapeTensorDataType#shape`  instance method.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.

* GitHub Issue: #46380

Lead-authored-by: Hiroyuki Sato <hiroysato@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: hiroyuki-sato

c_glib/arrow-glib/basic-data-type.cpp

@@ -2347,6 +2347,25 @@ garrow_fixed_shape_tensor_data_type_new(GArrowDataType *value_type,
   return data_type;
 }
 
+/**
+ * garrow_fixed_shape_tensor_data_type_get_shape:
+ * @data_type: A #GArrowFixedShapeTensorDataType.
+ * @length: (out): Return location for the number of dimensions of the tensor.
+ *
+ * Returns: (array length=length): Shape of the tensor.
+ */
+const gint64 *
+garrow_fixed_shape_tensor_data_type_get_shape(GArrowFixedShapeTensorDataType *data_type,
+                                              gsize *length)
+{
+  auto arrow_data_type = std::static_pointer_cast<arrow::extension::FixedShapeTensorType>(
+    garrow_data_type_get_raw(GARROW_DATA_TYPE(data_type)));
+
+  const auto &arrow_shape = arrow_data_type->shape();
+  *length = arrow_shape.size();
+  return arrow_shape.data();
+}
+
 G_END_DECLS
 
 GArrowDataType *

c_glib/arrow-glib/basic-data-type.h

@@ -826,4 +826,8 @@ garrow_fixed_shape_tensor_data_type_new(GArrowDataType *value_type,
                                         gsize n_dim_names,
                                         GError **error);
 
+GARROW_AVAILABLE_IN_21_0
+const gint64 *
+garrow_fixed_shape_tensor_data_type_get_shape(GArrowFixedShapeTensorDataType *data_type,
+                                              gsize *length);
 G_END_DECLS

c_glib/test/test-fixed-shape-tensor-data-type.rb

@@ -16,30 +16,29 @@
 # under the License.
 
 class TestFixedShapeTensorDataType < Test::Unit::TestCase
+  def setup
+    @data_type = Arrow::FixedShapeTensorDataType.new(Arrow::UInt64DataType.new,
+                                                     [3, 4],
+                                                     [1, 0],
+                                                     ["x", "y"])
+  end
+
   def test_id
-    data_type = Arrow::FixedShapeTensorDataType.new(Arrow::UInt64DataType.new,
-                                                    [3, 4],
-                                                    [1, 0],
-                                                    ["x", "y"])
-    assert_equal(Arrow::Type::EXTENSION, data_type.id)
+    assert_equal(Arrow::Type::EXTENSION, @data_type.id)
   end
 
   def test_name
-    data_type = Arrow::FixedShapeTensorDataType.new(Arrow::UInt64DataType.new,
-                                                    [3, 4],
-                                                    [1, 0],
-                                                    ["x", "y"])
     assert_equal(["extension", "arrow.fixed_shape_tensor"],
-                 [data_type.name, data_type.extension_name])
+                 [@data_type.name, @data_type.extension_name])
+  end
+
+  def test_shape
+    assert_equal([3, 4], @data_type.shape)
   end
 
   def test_to_s
-    data_type = Arrow::FixedShapeTensorDataType.new(Arrow::UInt64DataType.new,
-                                                    [3, 4],
-                                                    [1, 0],
-                                                    ["x", "y"])
     assert do
-      data_type.to_s.start_with?("extension<arrow.fixed_shape_tensor")
+      @data_type.to_s.start_with?("extension<arrow.fixed_shape_tensor")
     end
   end