feat: Add NVIDIA Eval integration

[JashG]

What does this PR do?

This PR adds support for NVIDIA's NeMo Evaluator API to the Llama Stack eval module. The integration enables users to evaluate models via the Llama Stack interface.

Test Plan

[Describe the tests you ran to verify your changes with result summaries. Provide clear instructions so the plan can be easily re-executed.]

1. Added unit tests and successfully ran from root of project: ./scripts/unit-tests.sh tests/unit/providers/nvidia/test_eval.py

tests/unit/providers/nvidia/test_eval.py::TestNVIDIAEvalImpl::test_job_cancel PASSED
tests/unit/providers/nvidia/test_eval.py::TestNVIDIAEvalImpl::test_job_result PASSED
tests/unit/providers/nvidia/test_eval.py::TestNVIDIAEvalImpl::test_job_status PASSED
tests/unit/providers/nvidia/test_eval.py::TestNVIDIAEvalImpl::test_register_benchmark PASSED
tests/unit/providers/nvidia/test_eval.py::TestNVIDIAEvalImpl::test_run_eval PASSED

2. Verified I could build the Llama Stack image: LLAMA_STACK_DIR=$(pwd) llama stack build --template nvidia --image-type venv

Documentation added to llama_stack/providers/remote/eval/nvidia/README.md

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