[EP ABI] support `Graph_GetModelMetadata` (#25768)
### Description
Add a new API `Graph_GetModelMetadata`
### Motivation and Context
VitisAI EP would convert ONNX IR to another IR which is suitable for AMD
AI compilers.
The metadata in a OrtModel contains many important infomation produced
by other tools, e.g. Olive.
This API potentially used by many other execution providers which need
to access the same information.
