Add add_halide_stub helper and build for linear_blur app.

[alexreinking]

This implements the bare minimum stub support necessary to build the linear_blur app. I tested it locally; the buildbots will pick it up since I added it to the main apps build in CMake.

Fixes #8735

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[alexreinking]

This might not be the best implementation, but it is just enough to get the app to build without in any way disrupting the existing build rules. We can think about ways to optimize the number of generator calls if stubs some day take off.