Ship the OpenVINO delegate as its own library

[shoumikhin]

The OpenVINO delegate was compiled into the Python extension, so only Python could
use it. A C++ application had no way to link it from the wheel, which is the same gap
the other delegates had before they were split out.

It now builds as its own library and ships beside the runtime, so an application can
ask for it:

find_package(executorch REQUIRED COMPONENTS backend_openvino)
target_link_libraries(my_app PRIVATE executorch::backend_openvino)

Only the adapter ships here. The OpenVINO runtime itself is loaded at run time and is
not part of the wheel, so the component's documentation says to install it separately,
and the wheel does not grow meaningfully: the adapter is a few kilobytes.

The library resolves the runtime from the shared runtime library rather than from the
static core, so the delegate registers into the one registry the process has instead of
a second private one.

Test Plan: built a wheel with OpenVINO enabled on Linux x86_64 and inspected it.
The library ships at executorch/lib/libexecutorch_backend_openvino.so. Its delegate
symbols moved out of the Python extension, 13 in the library and 0 in the extension,
so the process has one copy and one registration. Its only ExecuTorch dependency is
the shared runtime library, and it records no link dependency on OpenVINO, confirming
the runtime is still loaded at run time. Added an ownership row to the shared library
test so a future change that puts the delegate back into the extension fails there.

[shoumikhin]

Stack from ghstack (oldest at bottom):

• Name the Python extension after what it contains #21816
• Ship linkable libraries in the macOS wheel #21771
• -> Ship the OpenVINO delegate as its own library #21770

[pytorch-bot]

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