Python: provide methods to register single native function to the kernel (microsoft#2390)

### Motivation and Context

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solve microsoft#2321 

### Description

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### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

---------

Co-authored-by: Abby Harrison <[email protected]>
Co-authored-by: Abby Harrison <[email protected]>
Co-authored-by: Lee Miller <[email protected]>

Author: 3 people

python/semantic_kernel/kernel.py

@@ -133,6 +133,39 @@ def register_semantic_function(
 
         return function
 
+    def register_native_function(
+        self,
+        skill_name: Optional[str],
+        sk_function: Callable,
+    ) -> SKFunctionBase:
+        if not hasattr(sk_function, "__sk_function__"):
+            raise KernelException(
+                KernelException.ErrorCodes.InvalidFunctionType,
+                "sk_function argument must be decorated with @sk_function",
+            )
+        function_name = sk_function.__sk_function_name__
+
+        if skill_name is None or skill_name == "":
+            skill_name = SkillCollection.GLOBAL_SKILL
+        assert skill_name is not None  # for type checker
+
+        validate_skill_name(skill_name)
+        validate_function_name(function_name)
+
+        function = SKFunction.from_native_method(sk_function, skill_name, self.logger)
+
+        if self.skills.has_function(skill_name, function_name):
+            raise KernelException(
+                KernelException.ErrorCodes.FunctionOverloadNotSupported,
+                "Overloaded functions are not supported, "
+                "please differentiate function names.",
+            )
+
+        function.set_default_skill_collection(self.skills)
+        self._skill_collection.add_native_function(function)
+
+        return function
+
     async def run_stream_async(
         self,
         *functions: Any,

python/tests/unit/kernel_extensions/test_register_functions.py

@@ -0,0 +1,61 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+
+import pytest
+
+from semantic_kernel import Kernel
+from semantic_kernel.kernel_exception import KernelException
+from semantic_kernel.orchestration.sk_function_base import SKFunctionBase
+from semantic_kernel.skill_definition.sk_function_decorator import sk_function
+from semantic_kernel.skill_definition.skill_collection import SkillCollection
+
+
+def not_decorated_native_function(arg1: str) -> str:
+    return "test"
+
+
+@sk_function(name="getLightStatus")
+def decorated_native_function(arg1: str) -> str:
+    return "test"
+
+
+def test_register_valid_native_function():
+    kernel = Kernel()
+
+    registered_func = kernel.register_native_function(
+        "TestSkill", decorated_native_function
+    )
+
+    assert isinstance(registered_func, SKFunctionBase)
+    assert (
+        kernel.skills.get_native_function("TestSkill", "getLightStatus")
+        == registered_func
+    )
+    assert registered_func.invoke("testtest").result == "test"
+
+
+def test_register_undecorated_native_function():
+    kernel = Kernel()
+
+    with pytest.raises(KernelException):
+        kernel.register_native_function("TestSkill", not_decorated_native_function)
+
+
+def test_register_with_none_skill_name():
+    kernel = Kernel()
+
+    registered_func = kernel.register_native_function(None, decorated_native_function)
+    assert registered_func.skill_name == SkillCollection.GLOBAL_SKILL
+
+
+def test_register_overloaded_native_function():
+    kernel = Kernel()
+
+    kernel.register_native_function("TestSkill", decorated_native_function)
+
+    with pytest.raises(KernelException):
+        kernel.register_native_function("TestSkill", decorated_native_function)
+
+
+if __name__ == "__main__":
+    pytest.main([__file__])