[WebNN EP] Support external data (#22263)
### Description
This PR introduces support for registering external data inside WebNN
EP.
### Motivation and Context
- The WebNN EP needs to register the initializers at graph compilation
stage, for initializers from external data, it can't leverage the
general external data loader framework because the graph compilation of
WebNN EP is executed before external data loader called.
- Exposes the `utils::GetExternalDataInfo`, it is useful for WebNN EP to
read the external tensor's infomation.
- Define a new `registerMLConstant` in JSEP to create WebNN constants
from external data in WebNN backend, with the info of tensor as
parameters, as well as the `Module.MountedFiles`, which holds all
preloaded external files.
