Provide a way to fetch the file offset of the embedded data

[RaisinTen]

Problem

The current mechanism returns a pointer to the embedded data and its size. Passing it into V8 forces us to load the entire embedded data into a buffer in memory. It is not efficient to prematurely store the entire payload in memory, especially if the payload is huge.

See nodejs/node#42334 (comment).

Suggested solution

If the Postject Api returns the file offset of the embedded data, we will be able to parse only the parts that we are interested in.

[RaisinTen]

This is not needed anymore, see nodejs/node#42334 (comment)