[Feature Request] [Compile API] Add support for custom EPContext binary data writer

### Describe the feature request

**Problem**

Currently, `ModelCompilationOptions_SetEpContextEmbedMode` allows separating EPContext binary data from the model, but it strictly dumps this data into a separate file. This is problematic for WebNN implementation due to sandbox limitations that prevent direct file system writes.

**Possible Solution**

Custom Writer for EPContext:
Introduces a mechanism similar to `ModelCompilationOptions_SetOutputModelWriteFunc` and `ModelCompilationOptions_SetOutputModelGetInitializerLocationFunc`, allowing users to provide a custom callback function to handle the writing of EPContext binary data.

In-Memory Session Creation:
Adds support for creating an inference session directly from an in-memory model buffer combined with in-memory EPContext binary data.

### Describe scenario use case

This feature is needed for dumping model cache with size >2GB due to the limitation of protobuf.

Start by reading ModelCompilationOptions_SetEpContextEmbedMode, ModelCompilationOptions_SetOutputModelWriteFunc, and ModelCompilationOptions_SetOutputModelGetInitializerLocationFunc in the Compile API. Trace how EPContext data is currently written and how in-memory model buffers are used. Done means a custom EPContext writer and in-memory session creation support the WebNN sandbox use case and model caches larger than 2GB.

Written by the indexing model from the issue text.