Should data_entry types be hard-defined/constrained? Which ones?

[SharkWipf]

Related to discussion #1.
If we know exactly what kind of formats we're supporting, it would be helpful to lock in permitted file types to a set list.
Unfortunately, we don't really know what file types will be necessary in the future.

There are a lot of types we can easily predict.

• We can probably safely assume that any format will always functionally have a primary scene file at least. So we can make the existance of a scene0 (or scene1 if you prefer) data type mandatory.
• Scenes may or may not have a thumbnail. We can make a thumbnail use thumbnail0.
• Modern GS scenes tend to have a skybox. We can make that skybox0.
• Some formats may have an embedded mesh, or supporting pointclouds. mesh0, pointcloud0.
• Some formats may want to store the camera poses and/or input images along with the data for later review/processing, or even the entire optimizer state and/or checkpoint. camera0, image0, state0, checkpoint0?
• Some formats may have MLPs to store, or may have anchors to store. Some may want to store spherical harmonics in separate files. mlp0, anchors0, harmonics0?
• There's also "4d" formats with a time dimension, I assume those would have their own files to store.

How far do we have to go in this? We can predict some formats, but the field is very actively developing and new stuff keeps being introduced.
As far as I can see, we have 3 options here, and not necessarily mutually exclusive.

1. We could make an exhaustive list, and if anything more is needed, we'll need to update the spec. IMO this is the worst option.
2. We could add a few generic data types, like data or miscelaneous. It's all semantics anyway, we don't restrict what can be stored and how.
3. We could leave it open-ended. Define values for the formats we know are being used, making those standard for their purposes, but not restricting the spec to only those values.

Personally, I'm thinking the best solution is a bit of all 3:

• Add what we know of
• Add a few generic data types
• If new data types are being used in the future, update the spec to make them official.

Thoughts?