Add schema link for custom strong type connection (#835)

# Description

Add schema link for custom strong type connection.

**vscode extension create connection test:**

![image](https://github.com/microsoft/promptflow/assets/75061414/1f1acd44-b803-4b92-b23f-f93763049bfa)

**flow test with connection:**

![image](https://github.com/microsoft/promptflow/assets/75061414/07150c90-dc0a-40b1-9ea3-a0defe464381)


# All Promptflow Contribution checklist:
- [x] **The pull request does not introduce [breaking changes].**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
- [x] **I have read the [contribution guidelines](../CONTRIBUTING.md).**
- [ ] **Create an issue and link to the pull request to get dedicated
review from promptflow team. Learn more: [suggested
workflow](../CONTRIBUTING.md#suggested-workflow).**

## General Guidelines and Best Practices
- [x] Title of the pull request is clear and informative.
- [x] There are a small number of commits, each of which have an
informative message. This means that previously merged commits do not
appear in the history of the PR. For more information on cleaning up the
commits in your PR, [see this
page](https://github.com/Azure/azure-powershell/blob/master/documentation/development-docs/cleaning-up-commits.md).

### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

---------

Co-authored-by: cs_lucky <[email protected]>

src/promptflow/promptflow/_utils/connection_utils.py

@@ -37,6 +37,7 @@ def generate_custom_strong_type_connection_spec(cls, package, package_version):
 
 def generate_custom_strong_type_connection_template(cls, connection_spec, package, package_version):
     connection_template_str = """
+    $schema: https://azuremlschemas.azureedge.net/promptflow/latest/CustomStrongTypeConnection.schema.json
     name: "to_replace_with_connection_name"
     type: custom
     custom_type: {{ custom_type }}

src/promptflow/tests/executor/unittests/_core/test_tools_manager.py

@@ -167,6 +167,7 @@ def test_collect_package_tools_and_connections(self, install_custom_tool_pkg):
         }
 
         expected_template = {
+            "$schema": "https://azuremlschemas.azureedge.net/promptflow/latest/CustomStrongTypeConnection.schema.json",
             "name": "to_replace_with_connection_name",
             "type": "custom",
             "custom_type": "MyFirstConnection",
@@ -197,5 +198,4 @@ def test_collect_package_tools_and_connections(self, install_custom_tool_pkg):
 
         content = templates["my_tool_package.tools.my_tool_with_custom_strong_type_connection.MyCustomConnection"]
         expected_template_str = textwrap.dedent(expected_template)
-
-        assert content in expected_template_str
+        assert expected_template_str in content