Build flatcc for the host (#10855)

### Summary
* Similar to what we did with `flatc`, let's build `flatcc_cli` for the
host using
[ExternalProject](https://cmake.org/cmake/help/latest/module/ExternalProject.html)
* Move the `flatcc` definitions to be under `third-party/`
* Move the `etdump` and `bundled_program` definitions to be under their
respective folders
* Clean up the build files and remove redundant things

### Test plan
CI

```
$ ./install_executorch.sh

$ ./scripts/build_apple_frameworks.sh

$ rm -rf cmake-out \
   && cmake --preset macos-arm64 \
   && cmake --build cmake-out --parallel

$ rm -rf cmake-out \
   && cmake -DEXECUTORCH_BUILD_DEVTOOLS=ON -Dprotobuf_BUILD_TESTS=OFF -DEXECUTORCH_ENABLE_EVENT_TRACER=ON --preset macos-arm64 \
   && cmake --build cmake-out --parallel
```

cc @larryliu0820