mark binaries generated by bulk export with useful metadata.

[tadgh]

Currently, upon reading a binary resource, it is impossible to tell what generated it. This feature requests:

1. That the bulk export provider adds a new optional parameter, _exportId to its initiation methods, which will be used to mark resources generated by that bulk export.
2. that the WriteBinaryFiles step annotates its generated binaries with:
   • the provided _exportId if available.
   • the job ID of the job instance.
   • the resource type of the data contained within that binary.
3. Since Binaries do not support extensions, store this data in the binary.meta.extensions field.