[TRT RTX EP] EP context changes (#25747)
* Implements `GetEPContextNodes()`
* Enables usage of `AddExternalInitializersFromFilesInMemory` for models
that have to be communicated as byte stream but are larger than 2GB
* Add EP context unit tests for file, bytestreams and both embed modes
NOTE: For large models > 2GB, `embed_mode=0` must be used.
`embed_mode=1` fails due to protobuf limitations
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Co-authored-by: Maximilian Müller <maximilianm@nvidia.com>
