feat: convert image to grayscale (#366)

Closes #287

### Summary of Changes

* feat: Added method that returns converted grayscale image but leaves
the original unchanged.

---------

Co-authored-by: megalinter-bot <129584137+megalinter-bot@users.noreply.github.com>

src/safeds/data/image/containers/_image.py

@@ -222,6 +222,20 @@ def resize(self, new_width: int, new_height: int) -> Image:
         new_image._image = new_image._image.resize((new_width, new_height))
         return new_image
 
+    def convert_to_grayscale(self) -> Image:
+        """
+        Convert the image to grayscale.
+
+        Returns
+        -------
+        grayscale_image : Image
+            The grayscale image.
+        """
+        data = io.BytesIO()
+        grayscale_image = self._image.convert("L")
+        grayscale_image.save(data, format=self._format.value)
+        return Image(data, self._format)
+
     def crop(self, x: int, y: int, width: int, height: int) -> Image:
         """
         Return an image that has been cropped to a given bounding rectangle.

tests/safeds/data/image/containers/test_image.py

@@ -206,6 +206,22 @@ def test_should_return_resized_image(
         assert image.resize(new_width, new_height)._image.size == new_size
 
 
+class TestConvertToGrayscale:
+    @pytest.mark.parametrize(
+        ("image", "expected"),
+        [
+            (
+                Image.from_png_file(resolve_resource_path("image/snapshot_heatmap.png")),
+                Image.from_png_file(resolve_resource_path("image/snapshot_heatmap_grayscale.png")),
+            ),
+        ],
+        ids=["grayscale"],
+    )
+    def test_convert_to_grayscale(self, image: Image, expected: Image) -> None:
+        grayscale_image = image.convert_to_grayscale()
+        assert grayscale_image._image.tobytes() == expected._image.tobytes()
+
+
 class TestEQ:
     def test_should_be_equal(self) -> None:
         image = Image.from_png_file(resolve_resource_path("image/original.png"))