Update selective build example + CI for top-level targets

[GregoryComer]

Summary

Update the selective build example and CI to use the new top-level targets, where appropriate. I've positioned the example as showcasing two flows - a standard way, using top-level targets, and appropriate for most use cases, and an advanced flow where the user creates a custom kernel target. This is mainly useful when integrating custom ops, which we can't easily integrate with top-level targets due to the inverted dependency on the user build.

I've updated the CMake to take an additional arg to specify whether or not to define a custom target and refactored to use the standard top-level target when not. I've also unified the CMake option naming with the top-level target. Finally, I've updated the CI job to cover both flows.

Tracking task: #12948.

Test plan

Changes are covered by the test selective build job.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13741

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures

As of commit aa5a45c with merge base b4e1145 ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• Lint / lintrunner / linux-job (gh)
  >>> Lint for examples/selective_build/CMakeLists.txt:

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