Add support for GPU monitoring

[gjulianm]

What does this PR do?

This PR adds support for the GPU monitoring feature, doing all the required changes to improve experience of customers.

Motivation

Simplify deployment of GPU monitoring.

Additional Notes

This is an initial implementation of the feature. It does not support deployment of mixed clusters (those where not all nodes have GPUs).

Minimum Agent Versions

Are there minimum versions of the Datadog Agent and/or Cluster Agent required?

• Agent: v7.60.x for the GPU monitoring feature.
• Cluster Agent: N/A

Describe your test plan

1. Deploy the operator in a cluster
2. Deploy the agent resource with feature.gpu.enabled: yes.
3. Check that the deployed agent pod has runtimeClassName: nvidia with kubectl get pod datadog-agent-XXX -o json | jq ".spec.runtimeClassName".
4. Ensure that DD_GPU_MONITORING_ENABLED is set to true in both the agent and system-probe containers.

Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• PR has a milestone or the qa/skip-qa label

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.00000% with 20 lines in your changes missing coverage. Please review.

Project coverage is 48.96%. Comparing base (db00883) to head (da4ab24).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...nal/controller/datadogagent/feature/gpu/feature.go	90.09%	9 Missing and 1 partial ⚠️
internal/controller/testutils/agent.go	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1601      +/-   ##
==========================================
+ Coverage   48.94%   48.96%   +0.01%     
==========================================
  Files         227      236       +9     
  Lines       20636    21315     +679     
==========================================
+ Hits        10101    10437     +336     
- Misses      10010    10337     +327     
- Partials      525      541      +16     

Flag	Coverage Δ
unittests	48.96% <84.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
api/datadoghq/v2alpha1/datadogagent_types.go	100.00% <ø> (ø)
internal/controller/datadogagent/controller.go	51.85% <ø> (ø)
...ller/datadogagent/defaults/datadogagent_default.go	91.24% <100.00%> (+0.14%)	⬆️
pkg/testutils/builder.go	91.62% <100.00%> (+0.10%)	⬆️
...nal/controller/datadogagent/feature/gpu/feature.go	90.09% <90.09%> (ø)
internal/controller/testutils/agent.go	0.00% <0.00%> (ø)

... and 11 files with indirect coverage changes

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[buraizu]

Approving with a minor edit requested