Add `morphedByMany` as relation

### Discussed in https://github.com/CodeDredd/pinia-orm/discussions/1889

<div type='discussions-op-text'>

<sup>Originally posted by **jhercog** July  8, 2024</sup>
Couldn't find any information about it in the docs or on the GitHub repo. Vuex-ORM had a `morphedByMany()` attribute. Does Pinia ORM have an equivalent, or perhaps something similar?</div>