Experimental C API support (#28746)
### Description
This pull request introduces a mechanism for exposing experimental C API
functions in ONNX Runtime. The new system enables the addition,
iteration, and eventual promotion of experimental APIs without impacting
the stable ABI, using a name-based function pointer lookup and a
generated header for type safety and ergonomics. The changes include
documentation, build integration, header generation, implementation, and
test coverage for the new experimental API flow.
**Experimental C API Framework**
* Added a design doc (`Experimental_C_API.md`) detailing the motivation,
design decisions, and usage patterns for the experimental C API
mechanism.
* Introduced a central declaration file
(`onnxruntime_experimental_c_api.inc`) using X-macros to define
experimental API functions and their lifecycle rules. The X-macro
signature uses `ORT_EXPERIMENTAL_API(VER, RET, NAME, ...)` ordering
(return type before name) to match the convention used by `ORT_API_T` in
the stable API.
* Added a generated consumer header (`onnxruntime_experimental_c_api.h`)
that provides C typedefs, name constants, and C++ typed accessors for
experimental functions. Experimental function names follow the pattern
`<TargetStruct>_<Name>_SinceV<APIVersion>` to unambiguously convey
availability and avoid collision.
* Updated the stable C API struct (`OrtApi` in `onnxruntime_c_api.h`) to
include a single function pointer, `GetExperimentalFunction`, for
name-based experimental function lookup. The `OrtExperimentalFnPtr`
generic function pointer type (rather than `void*`) is used as the
return type to avoid undefined behavior when casting between function
pointers.
* Integrated the new headers into the build system so they are installed
and available to consumers.
**Implementation and Test Coverage**
* Implemented the runtime support for experimental API lookup and
function registration (`experimental_c_api.cc`), including a test-only
function (`OrtApi_ExperimentalApiTest`) to exercise the mechanism
end-to-end.
* Registered the new experimental API entry point in the exported API
table (`onnxruntime_c_api.cc`).
* Added a unit test source file for experimental API coverage.
### Motivation and Context
Enable support for experimental C APIs.
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
