agent: add DisableNvidia config switch

[Anson Qian (anson627)]

This pull request introduces a new DisableNvidia option to the agent configuration, allowing users to explicitly disable NVIDIA GPU detection and runtime setup. The implementation ensures that when DisableNvidia is set to true, all related NVIDIA functionality is bypassed, even if NVIDIA devices are present on the host. The change is thoroughly tested and documented.

[Philip Lombardi (plombardi89)]

I feel like DisableNvidia is a bit too simplistic of a flag. What if we had a dedicated configuration block for Nvidia or really any other GPU driver wanted to support, for example:

{
  "gpu": {
    "kind": "amd", "nvidia", "intel", "none" // none used when someone wants to do their own thing. `none` would also the be the default if `gpu` block is null.
    "nvidia": { /* nvidia customization options */ }
    "amd": { /* amd customization options */ }
    "intel": ... same pattern ...
  }
}