Add function to return session handle from nisync (ni#18)

Signed-off-by: wchung <[email protected]>

- [x] This contribution adheres to
[CONTRIBUTING.md](https://github.com/ni/nisync-python/blob/main/CONTRIBUTING.md).

### What does this Pull Request accomplish?

Add a function that return read-only session handle.

### Why should this Pull Request be merged?

Existing function that returning session object cannot fulfill
NI-Digital use case.

### What testing has been done?
PR Build 
[manual pytest]
1. Added and ran a new regression test for session_handle function.
2. Ran command "poetry run ni-python-styleguide lint" and "poetry run
ni-python-styleguide fix" to correct the code format as one of the
requirement in CONTRIBUTING.md.

![image](https://github.com/user-attachments/assets/2f80b625-1a85-4654-ab52-324a27218467)

---------

Signed-off-by: wchung <[email protected]>

Author: wchung-ni

nisync/session.py

@@ -864,6 +864,10 @@ def time_references(self):
     def resource_name(self):
         return self._resource_name
 
+    @property
+    def session_handle(self):
+        return self._vi
+
 
 class TimeReference(object):
     def __init__(self, session):

templates/nisync/session.py.mako

@@ -649,6 +649,11 @@ class Session(_SessionBase):
     @property
     def resource_name(self):
         return self._resource_name
+    
+    @property
+    def session_handle(self):
+        return self._vi
+
 <%
     class_hierarchy = [
         ('TimeReference',           'object'),