Move examples + tests to single-step CMake build

[GregoryComer]

🚀 The feature, motivation and pitch

Currently, many examples and tests are built using a two-step process, where the executorch core is built and then "installed" into the build directory used by the example or test. This ends up being quite fragile as iterative rebuilds often don't work nicely and it's very easy for it not work when the initial build isn't configured exactly right for the test or example. It should be significantly more robust and "just work" if we either migrate all of these targets to include executorch via add_subdirectory (it should be possible to include the parent directory, so long as there is no cycle. Might need to play around with relative vs absolute paths). The other option is to include them from the main build/CMakeLists under the main CMakeLists. It may make sense to do this for tests and keep examples using add_subdirectory, but could be discussed more.

Alternatives

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Additional context

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RFC (Optional)

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cc @larryliu0820 @mergennachin @byjlw