This new file can parse the file header (first few bytes) as well as the block
headers.
Right now, this is used by two places:
* `blendfile.py` which is used by `blend2json.py`
* `blend_render_info.py`
This new module is shipped with Blender because it's needed for
`blend_render_info.py` which is shipped with Blender too. This makes using it in
`blendfile.py` (which is not shipped with Blender) a bit more annoying. However,
this is already not ideal, because e.g. `blend2json` also has to add to
`sys.path` already to be able to import `blendfile.py`.
This new file could also be used by blender-asset-tracer (BAT).
The new `BlendFileHeader` and `BlockHeader` types may be subclassed by code
using it, because it wants to store additional derived data (`blendfile.py` and
BAT need this).
New tests have been added that check that the file and block header is parsed
correctly for different kinds of .blend files.
Pull Request: https://projects.blender.org/blender/blender/pulls/140341
