Selectors for parent collection cache

It would be an interesting addition to be able to provide a selector for the parent collection. For example:

```js
// Only add a chairCount on furniture of type "table"
Furniture.cacheCount({
collection: Furniture,
referenceField: 'furnitureLink',
cacheField: 'chairCount',
parentSelector: { type: 'table' },
selector: { type: 'chair' },
})

// Only add a legCount on furniture of type "chair"
Furniture.cacheCount({
collection: Legs,
referenceField: 'furnitureLink',
cacheField: 'legCount',
parentSelector: { type: 'chair' },
})
```

This allows "polymorphic" collections to have different caches depending on their category, type, etc.

It is mostly done for keeping things clean, but one could also imagine using different cache configurations with the same name:

Here's a sarcastic example 🤣:
```js
Users.cacheCount({
collection: Users,
referenceField: 'friendIds',
cacheField: 'americanFriends',
parentSelector: { citizenShip: 'american' },
selector: { citizenShip: 'american' },
})

Users.cacheCount({
collection: Users,
referenceField: 'friendIds',
cacheField: 'americanFriends',
parentSelector: { citizenShip: 'european' },
selector: { citizenShip: { $in: ['north-american', 'south-american', 'american'] } },
})
```

You would be responsible for maintaining non-overlapping parent selectors of course, otherwise it will not work.

No file or test is named in the issue. Start by tracing the existing Furniture.cacheCount API and its handling of selector and parent relationships; review how current cache configurations are represented, then define completion as supporting non-overlapping parentSelector configurations alongside the shown selector behavior.

Written by the indexing model from the issue text.