Creating `SyntheticField` across subgraphs

**Is your feature request related to a problem? Please describe.**
Being able to seamlessly combine fields from multiple subgraphs via `SyntheticField`.

**Describe the solution you'd like**
```py
borrows = aave.Borrows + compound.Borrows
```

*Originally from: ThierryBleau*

Look at the existing SyntheticField implementation in the codebase, likely in a module like subgrounds/synthetic.py. Understand how subgraphs like aave and compound are currently queried and how their Borrows fields are structured. The goal is to design a mechanism to combine fields across different subgraph schemas, which requires understanding GraphQL stitching and the library's internal data models.

索引モデルが issue の本文から書いたものです。