[EP ABI] Add weight pre-packing support to kernel-based plugin EPs

[adrianlizarraga]

Description

• Adds C APIs to support pre-packing of const weights for OrtKernelImpl implementations.
  • APIs optionally support sharing of pre-packed weight data (for cpu-accessible memory).
• Updates example kernel (Mul) to use new pre-packing API. Tested by existing unit test:

  onnxruntime/onnxruntime/test/autoep/test_execution.cc

  Lines 242 to 256 in 549d741

  	TEST(OrtEpLibrary, KernelPluginEp_Inference) {
  	RegisteredEpDeviceUniquePtr example_kernel_ep;
  	ASSERT_NO_FATAL_FAILURE(Utils::RegisterAndGetExampleEp(*ort_env, Utils::example_ep_kernel_registry_info,
  	example_kernel_ep));
  	Ort::ConstEpDevice plugin_ep_device(example_kernel_ep.get());
  	// Create session with example kernel-based plugin EP
  	Ort::SessionOptions session_options;
  	session_options.AddConfigEntry(kOrtSessionOptionsDisableCPUEPFallback, "1"); // Fail if any node assigned to CPU EP.
  	std::unordered_map<std::string, std::string> ep_options;
  	session_options.AppendExecutionProvider_V2(*ort_env, {plugin_ep_device}, ep_options);
  	// This model has Squeeze, Mul, and Relu nodes. The example plugin EP supports all nodes using registered kernels.
  	Ort::Session session(*ort_env, ORT_TSTR("testdata/squeeze_mul_relu.onnx"), session_options);

Motivation and Context

The previous PR added the base APIs that support kernel-based plugin EPs. This PR adds an additional feature that was identified as necessary for the port of WebGPU EP.

[fs-eire]

Approve for the API and examples.