Address security issue of loading arbitrary files as external data (#26776)
### Description
Verify external data references in TensorProto specify data location
that is under the model directory structure, reject absolute paths and
paths that escape the model path.
Make the validation function available to bridge based EPs.
Expose ExternalDataInfo via a bridge to some EPs that choose to handle
the data itself.
### Motivation and Context
This is a security concern.