Add substitution capability that doesn't require a virtual filesystem or a filename

[Skarlso]

What would you like to be added:

Ability to perform substitution on plain byte data.

Why is this needed:

The substitution functionality right now requires a VFS and files to be passed in. But in same cases, when we only have a blob, we can't pass in a filename. And since libraries aren't aware of Acces methods ( which are raw JSON ), we can't have a path information to fetch the name of the file to create a tar file ourselves.