feat(earn): add function for fetching aave rewards (#5422)

### Description

As the title

### Test plan

Unit tests

### Related issues

- Part of ACT-1180

### Backwards compatibility

Yes

### Network scalability

If a new NetworkId and/or Network are added in the future, the changes
in this PR will:

- [x] Continue to work without code changes, OR trigger a compilation
error (guaranteeing we find it when a new network is added)