Add support for hashing files with header.

[mihaimaruseac]

Summary

Missed this in #188, but found out I need it when working on #190. The serialize_v0/serialize_v1 methods all had headers in front of the files, so we need to do that too. Will update usage of header on #190 shortly.

As a benefit, we can simulate hashing a file with a header for the first portion of the file and a sharded hasher for the remainder of the file.

Release Note

NONE

Documentation

NONE

[laurentsimon]

It's a bit dangerous that:
"header" + "file_content" will return the same hash as "head" + "erfile_content"

What's the use case?

[mihaimaruseac]

Actually, I don't need it.

In the old serialization we had

        h = hashlib.sha256(header)
        with open(path, "rb") as f:
            if chunk == 0:
                all_data = f.read()
                h.update(all_data)
            else:
                # Compute the hash by reading chunk bytes at a time.
                while True:
                    chunk_data = f.read(chunk)
                    if not chunk_data:
                        break
                    h.update(chunk_data)
        return h.digest()

So we needed a header before hashing the file. But we actually have 2 different hashers here, so this is not needed.