Add support for secure Mattermost notifications

Apprise supports this but we need to surface the configuration part.
Keeping default to insecure connection to not break compatibility, but
perhaps worth reconsidering.

src/prefect/blocks/notifications.py

@@ -570,6 +570,10 @@ class MattermostWebhook(AbstractAppriseNotificationBlock):
         description="The hostname of your Mattermost server.",
         examples=["Mattermost.example.com"],
     )
+    secure: bool = Field(
+        default=False,
+        description="Whether to use secure https connection.",
+    )
 
     token: SecretStr = Field(
         default=...,
@@ -621,6 +625,7 @@ def block_initialization(self) -> None:
                 channels=self.channels,
                 include_image=self.include_image,
                 port=self.port,
+                secure=self.secure,
             ).url()  # pyright: ignore[reportUnknownMemberType, reportUnknownArgumentType] incomplete type hints in apprise
         )
         self._start_apprise_client(url)

tests/blocks/test_notifications.py

@@ -170,6 +170,30 @@ async def test_notify_async(self):
                 body="test", title="", notify_type=PREFECT_NOTIFY_TYPE_DEFAULT
             )
 
+    def test_notify_secure(self):
+        with patch("apprise.Apprise", autospec=True) as AppriseMock:
+            apprise_instance_mock = AppriseMock.return_value
+            apprise_instance_mock.async_notify = AsyncMock()
+
+            mm_block = MattermostWebhook(
+                hostname="example.com", token="token", secure=True, port=443
+            )
+
+            @flow
+            def test_flow():
+                mm_block.notify("test")
+
+            test_flow()
+
+            AppriseMock.assert_called_once()
+            apprise_instance_mock.add.assert_called_once_with(
+                f"mmosts://{mm_block.hostname}/{mm_block.token.get_secret_value()}/"
+                "?image=no&format=text&overflow=upstream"
+            )
+            apprise_instance_mock.async_notify.assert_called_once_with(
+                body="test", title="", notify_type=PREFECT_NOTIFY_TYPE_DEFAULT
+            )
+
     def test_notify_sync(self):
         with patch("apprise.Apprise", autospec=True) as AppriseMock:
             apprise_instance_mock = AppriseMock.return_value