[QNN EP] Add support for Mean Op in QNN EP (#25021)
- Implemented MeanOpBuilder to support ONNX Mean operator in QNN EP.
- Decomposed Mean into a sequence of element-wise Add operations followed by a Div Op.
- Added unit tests for Mean op running on HTP
### Description
Adds support for the ONNX Mean operator in QNN EP via Add + Div decomposition.
### Motivation and Context
Enables execution of models using Mean on QNN backend, improving Op
support.
