Move OrtModelPackageApi to the experimental C API

[jambayk]

Description

Move the existing model package C API off the stable OrtApi onto the experimental name-based lookup mechanism added in #28746. Each model package function is registered individually in include/onnxruntime/core/session/onnxruntime_experimental_c_api.inc with the OrtModelPackageApi_ prefix and the _SinceV28 version suffix, following the lifecycle rules in docs/design/Experimental_C_API.md.

Headline changes:

• OrtApi::GetModelPackageApi, the OrtModelPackageApi struct, OrtApis::GetModelPackageApi, the OrtModelPackageAPI namespace, onnxruntime/core/session/model_package_api.h, and the C++ wrappers (Ort::GetModelPackageApi, ORT_DEFINE_RELEASE_FROM_API_STRUCT(ModelPackage*), ModelPackageOptions/Context/ComponentContext) are removed.
• Opaque handle types (OrtModelPackageOptions, OrtModelPackageContext, OrtModelPackageComponentContext) move into onnxruntime_experimental_c_api.h.
• All 15 model package functions are registered in onnxruntime_experimental_c_api.inc. Impls move into namespace OrtExperimentalApis with _SinceV28-suffixed names in model_package_api.cc; bodies are unchanged.
• experimental_c_api.cc gains a forward-decl block (driven by the same .inc X-macro) so the auto-generated registration table can take the address of every entry, even those defined in model_package_api.cc.
• The Python bindings (PyModelPackageContext / PyModelPackageOptions / PyModelPackageComponentContext and their onnxruntime.__init__ exports) are removed. Per the design doc we start the experimental API in C/C++ only.
• onnxruntime/test/autoep/test_model_package.cc switches to a local ModelPackageFns struct populated through the Ort::Experimental::Get_OrtModelPackageApi_*_Fn(api) typed accessors.

Consumer usage going forward, in C++:

#include "onnxruntime_c_api.h"
#include "onnxruntime_experimental_c_api.h"

const OrtApi* ort = OrtGetApiBase()->GetApi(ORT_API_VERSION);

if (auto* fn = Ort::Experimental::Get_OrtModelPackageApi_CreateModelPackageContext_SinceV28_Fn(ort)) {
  OrtModelPackageContext* ctx = nullptr;
  Ort::ThrowOnError(fn(ORT_TSTR("/path/to/pkg"), &ctx));
  // ...
}

Motivation and Context

The model package API was added to the stable OrtApi in 1.27 but has not shipped in a release yet. Now that #28746 has landed the experimental C API framework, the right home for an iterating preview surface like model package is behind OrtApi::GetExperimentalFunction, not on the stable struct.

Moving it to experimental:

• frees us to change signatures (each name is uniquely versioned) without breaking the stable ABI;
• gives consumers a clear "is this specific thing available?" contract instead of a struct that looks stable but isn't;
• lets the surface be promoted to stable cleanly later (move entries to OrtApi, drop the _SinceV<N> suffix, remove the experimental entries).

[edgchen1]

• The Python bindings (PyModelPackageContext / PyModelPackageOptions / PyModelPackageComponentContext and their onnxruntime.__init__ exports) are removed. Per the design doc we start the experimental API in C/C++ only.

it is a starting point. if we have a need for exposing experimental APIs in Python, we can explore how to do that.