Generalize image source interface for pure Python implementations

@thewtex wrote in https://github.com/InsightSoftwareConsortium/itk-dreg/pull/2/files#r1354873358 :

> Let's go with this for now, and continue integrating the prototype implementation, however, I think we should change the return type to be an interface that can be implemented in pure Python. Internally we can use itk.ImageFileReader for some implementations. Most likely an OME-Zarr based interface. This way, alternative implementations can easily be created. And we have something that can be used in Pyodide.

We should define a virtual Python interface such as `StreamableImageSource` that defines operations to get the image 1) with only metadata and/or 2) with a streamed voxel subregion.