[EP ABI] Add support for creating EP Context models. (#25124)
### Description
- Updates `OrtEp::Compile()` to allow a plugin EP to create and return
EPContext nodes.
- Updates the example EP plugin to generate an example EPContext model:
<img width="747" alt="image"
src="https://github.com/user-attachments/assets/e5d98a10-ec15-45aa-bfaf-887d3b6226e2"
/>
### Motivation and Context
Adds more of the functionality missing from the EP ABI used for plugin
EPs.
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Edward Chen <18449977+edgchen1@users.noreply.github.com>