Function readHeader

Reads and validates the file preamble and header, in this order:

1. Read the first 8 bytes and validate the magic.
2. Read the 4-byte size prefix and validate it against MAX_HEADER_SIZE and the file's total size.
3. Read [8, 12 + headerSize) -- the prefix INCLUDED -- and hand that slice to Header.getSizePrefixedRootAsHeader.
4. Build FileInfo off the parsed header.
5. Compute the layout with computeLayout and validate it with validateLayoutAgainstSize.
6. Fill in each attribute index's absolute begin offset.