Key for marking an attribute as runtime-initialized
I am currently dissecting a lot of game assets where certain tables get initialized/patched at runtime, by the game itself.
Most of the time, the values are pointers to other assets or other parts of the same file. I assume this made loading assets more straightforward, because there is no need to allocate additional memory if the assets already have space carved out for runtime fields.

Right now, I've been annotating this through ordinary comments and sometimes doc keys, but it would feel nicer to have an actual key for this. It could look like this:

```yaml
- id: texture_info_ptr
  type: u4
  runtime: true
```
