[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/modules/utils.py

@@ -4,6 +4,7 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+import base64
 import os
 
 import pandas as pd
@@ -29,3 +30,13 @@ def process_dataset(file):
     except Exception as e:
         st.error(f"Error processing file: {str(e)}")
         return None
+
+
+def data_url_from_file(file) -> str:
+    file_content = file.getvalue()
+    base64_content = base64.b64encode(file_content).decode("utf-8")
+    mime_type = file.type
+
+    data_url = f"data:{mime_type};base64,{base64_content}"
+
+    return data_url