Python: Introduce feature decorator to allow for experimental and release candidate decorator usage (microsoft#10691)

### Motivation and Context

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This change is required to improve the flexibility and maintainability
of our feature annotation system. Previously, separate decorators (e.g.,
`experimental_function` and `experimental_class`) were used to mark
experimental features, resulting in code duplication and limiting our
ability to handle additional feature stages. As our SDK evolves, we need
a unified approach that can support multiple stages—such as
experimental, release candidate, and future states—while also allowing
version information for release candidate features to be centrally
managed.

### Description

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These notes will help understanding how your code works. Thanks! -->

This PR refactors our feature decorators by introducing a unified
`stage` decorator that updates the docstring and attaches metadata for
both functions and classes. Two convenience decorators, `experimental`
and `release_candidate`, are built on top of `stage`:
- The `experimental` decorator marks features as experimental and sets
an `is_experimental` attribute.
- The `release_candidate` decorator supports multiple usage patterns
(with or without parentheses and with an optional version parameter) to
mark features as release candidate and sets an `is_release_candidate`
attribute.

This unified approach reduces duplication, simplifies the codebase, and
lays the groundwork for easily extending feature stages in the future.

This decorator supports the following usage patterns:
- `@experimental` (for both classes and functions)
- `@release_candidate`  (no parentheses)
- `@release_candidate()`  (empty parentheses)
- `@release_candidate("1.21.3-rc1")`  (positional version)
- `@release_candidate(version="1.21.3-rc1")`  (keyword version)

### Contribution Checklist

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- [X] The code builds clean without any errors or warnings
- [X] 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
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

Author: moonbox3

python/samples/demos/document_generator/custom_selection_strategy.py

@@ -12,7 +12,7 @@
 )
 from semantic_kernel.connectors.ai.open_ai.services.open_ai_chat_completion import OpenAIChatCompletion
 from semantic_kernel.contents.chat_history import ChatHistory
-from semantic_kernel.utils.experimental_decorator import experimental_class
+from semantic_kernel.utils.feature_stage_decorator import experimental
 
 if TYPE_CHECKING:
     from semantic_kernel.agents import Agent
@@ -21,7 +21,7 @@
 NEWLINE = "\n"
 
 
-@experimental_class
+@experimental
 class CustomSelectionStrategy(SelectionStrategy):
     """A selection strategy that selects the next agent intelligently."""
 

python/semantic_kernel/__init__.py

@@ -3,4 +3,7 @@
 from semantic_kernel.kernel import Kernel
 
 __version__ = "1.21.3"
-__all__ = ["Kernel", "__version__"]
+
+DEFAULT_RC_VERSION = f"{__version__}-rc1"
+
+__all__ = ["DEFAULT_RC_VERSION", "Kernel", "__version__"]