move playground ui to llama-stack repo (#536)

# What does this PR do?

- Move Llama Stack Playground UI to llama-stack repo under
llama_stack/distribution
- Original PR in llama-stack-apps:
https://github.com/meta-llama/llama-stack-apps/pull/127

## Test Plan
```
cd llama-stack/llama_stack/distribution/ui
streamlit run app.py
```


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/distribution/ui/README.md

@@ -0,0 +1,11 @@
+# LLama Stack UI
+
+[!NOTE] This is a work in progress.
+
+## Running Streamlit App
+
+```
+cd llama_stack/distribution/ui
+pip install -r requirements.txt
+streamlit run app.py
+```