[QNN EP] Add UDO reference sample scripts

[huaychou]

Description

Add a user-facing MyAdd QNN user-defined operation (UDO) reference sample at qcom/samples/qnn_udo_myadd/. The sample demonstrates the complete QNN EP custom-op workflow outside the unit-test binary:

• Generate fp32 (CPU) and QDQ (HTP) ONNX models, plus deterministic ONNX test data.
• Build Linux x86_64 CPU and HTP op packages using the canonical MyAdd op-package sources shared with the unit tests. Generated output is isolated under the sample's ignored artifacts/ directory.
• Run the models through QNN EP using standalone C++ and Python plugin-EP examples.
• Deploy and run the HTP UDO on Android arm64, including the required ARM registration library and DSP skel arrangement.
• Generate an external EPContext model and run it without the custom-op schema environment variable.
• Register the custom ONNX domain through ORT_QNN_CUSTOM_OP_DOMAINS; no manual Ort::CustomOpDomain setup or schema companion library is required.
• Link the QNN EP UDO documentation to the sample.

Motivation and context

The MyAdd UDO coverage previously lived primarily in unit-test fixtures. This sample provides a copy-runnable developer reference for building, registering, validating, and deploying a QNN UDO across CPU, HTP simulation, and Android HTP execution.

Validation

• Linux x86_64 CPU: built the op package; C++ and Python samples passed with maximum absolute error 0.00e+00.
• Linux x86_64 HTP simulator: built the op package; C++ and Python samples passed with maximum absolute error 0.0118 (within the 0.0314 QDQ tolerance).
• Linux x86_64 gtests: QnnCPUBackendTests.UDO_Op_MyAdd_AutoDomainOnly and QnnHTPBackendTests.UDO_Op_MyAdd passed.
• Android arm64 HTP: inference passed (Models: 1, Succeeded: 1, Failed: 0).
• Android arm64 EPContext: external context generation and inference passed without ORT_QNN_CUSTOM_OP_DOMAINS.

[yuhuchua-qti]

The PR description references register_myadd_schema.cc, but this file is not part of the PR.
The updated README/sample flow also appears to rely on ORT_QNN_CUSTOM_OP_DOMAINS instead of a schema companion library.
Could you clarify this discrepancy?
Thanks.

[huaychou]

The PR description references register_myadd_schema.cc, but this file is not part of the PR. The updated README/sample flow also appears to rely on ORT_QNN_CUSTOM_OP_DOMAINS instead of a schema companion library. Could you clarify this discrepancy? Thanks.

After #770, the manual UDO custom-op domain registration is no longer required, so register_myadd_schema.cc has been removed.
Update the PR description according to the latest changes and validation results.