Feat : Doc Enhancement - Add README for dice_agent_rest Python sample

### Is your feature request related to a problem? Please describe.

The samples/python/agents/dice_agent_rest folder doesn’t currently have a README.

### Describe the solution you'd like

I’d like to add a README covering prerequisites, configuration (DICE_AGENT_URL), and how to run the REST dice client, including how it relates to dice_agent_grpc.

If this sounds good, I can open a PR with the new README.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

The README is needed in the samples/python/agents/dice_agent_rest folder. Examine the existing dice_agent_grpc README for structure and content. The new README should cover prerequisites, setting DICE_AGENT_URL, running the REST client, and its relation to the gRPC version. Verify by checking the folder's current files and ensuring the instructions work.

Vom Indexierungsmodell aus dem Issue-Text verfasst.