### What?
This PR adds ability to define indexes on several fields for collections
(compound indexes).
Example:
```ts
{
indexes: [{ unique: true, fields: ['title', 'group.name'] }]
}
```
### Why?
This can be used to either speed up querying/sorting by 2 or more fields
at the same time or to ensure uniqueness between several fields.
### How?
Implements this logic in database adapters. Additionally, adds a utility
`getFieldByPath`.
bacc0f002a