FEAT: Add py.typed marker (#367)

Fixes #359

### Summary
- add PEP 561 py.typed marker so type checkers see bundled stubs
- include marker in package_data for distribution

### Rationale
- fixes missing typing marker reported in #359 to avoid linter warnings
after install

### Changes
- add mssql_python/py.typed
- ship py.typed via setup.py package_data

---------

Co-authored-by: Gaurav Sharma <sharmag@microsoft.com>

Author: cnaples79

mssql_python/py.typed

@@ -0,0 +1 @@
+

setup.py

@@ -107,6 +107,7 @@ def finalize_options(self):
     package_data={
         # Include PYD and DLL files inside mssql_python, exclude YML files
         "mssql_python": [
+            "py.typed",  # Marker file for PEP 561 typing support
             "ddbc_bindings.cp*.pyd",  # Include all PYD files
             "ddbc_bindings.cp*.so",  # Include all SO files
             "libs/*",