Add option to set desired GPU count

[hlts2]

This PR modifies the application to allow setting the desired GPU count via an functional option.
Since having a variable in the New function about gpu count makes it look like a required parameter, it is better to set it via an option instead.

Summary

If the desired GPU count is not set via environment variables, it defaults to 0.
When the desired GPU count is 0, GPU count checking is skipped.
This enables node-agent to run in both GPU and non-GPU environments.

Behavior

• GPU environments:

  • The existing two conditions for checks remain unchanged:
    • The node enters the NotReady state.
    • The number of available GPUs per node falls below a configured threshold.

• Non-GPU environments:

  • GPU count checking is skipped.
  • Only the NotReady state condition is evaluated.

This enhancement improves flexibility by supporting both GPU-equipped and non-GPU environments seamlessly.

This is not urgent, so it's fine to include it with the next release.

[hlts2]

@johndietz Thank you for your review🙏I will merge this PR🚀