feat: Add artifact metadata support and a new sample for context offloading

Enhanced `save_artifact` in `callback_context.py` to accept `custom_metadata` and added `get_artifact_version` to retrieve artifact details.

Introduced a new sample, `context_offloading_with_artifact`, demonstrating how to use ADK artifacts to offload large data from the LLM context. The sample includes:
-   `QueryLargeDataTool`: Generates mock sales reports, saves them as artifacts with custom metadata, and injects the artifact content into the LLM request immediately after creation.
-   `CustomLoadArtifactsTool`: Provides summaries of available artifacts to the LLM based on metadata and loads artifact content on demand when `load_artifacts` is called.

Co-authored-by: Hangfei Lin <hangfei@google.com>
PiperOrigin-RevId: 830592786

tests/unittests/agents/test_callback_context.py

@@ -296,6 +296,7 @@ class TestCallbackContext:
         session_id="test-session-id",
         filename="test_file.txt",
         artifact=test_artifact,
+        custom_metadata=None,
     )
     assert version == 1