[llama stack ui] add native eval & inspect distro & playground pages (#541)

# What does this PR do?

New Pages Added: 

- (1) Inspect Distro
- (2) Evaluations: 
  - (a) native evaluations (including generation)
  - (b) application evaluations (no generation, scoring only)
- (3) Playground: 
  - (a) chat
  - (b) RAG  

## Test Plan

```
streamlit run app.py
```

#### Playground

https://github.com/user-attachments/assets/6ca617e8-32ca-49b2-9774-185020ff5204

#### Inspect

https://github.com/user-attachments/assets/01d52b2d-92af-4e3a-b623-a9b8ba22ba99


#### Evaluations (Generation + Scoring)

https://github.com/user-attachments/assets/345845c7-2a2b-4095-960a-9ae40f6a93cf

#### Evaluations (Scoring)

https://github.com/user-attachments/assets/6cc1659f-eba4-49ca-a0a5-7c243557b4f5


## 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

@@ -2,6 +2,12 @@
 
 [!NOTE] This is a work in progress.
 
+## Prerequisite
+- Start up Llama Stack Server
+```
+llama stack run
+```
+
 ## Running Streamlit App
 
 ```