store masks in geff as bool, not uint64

[TeunHuijben]

In our original understanding, geff has the requirement that variable length attributes must be stored as unit64.

However, there is nuance to this. A variable-length attribute has values (indices) and data (the actual data). Geff only enforced the values to be unit64, not the data! So it is perfectly fine for us to store mask as bool, as long as the values are still unit64.

Saving the masks as bool saves us a lot of memory (64x). Not for the on-disc geff, because the zarr compression is the same for uint64/bool, but for loading and in-memory graphs, this is a huge win.

This PR is a draft to see if it works.

Any ideas, @JoOkuma?

[JoOkuma]

@TeunHuijben, I'm surprised this worked.
If I recall correctly, at some point, this would fail with the geff round-trip tests.
LGTM!