### What?
For the join field query adds ability to specify `count: true`, example:
```ts
const result = await payload.find({
joins: {
'group.relatedPosts': {
sort: '-title',
count: true,
},
},
collection: "categories",
})
result.group?.relatedPosts?.totalDocs // available
```
### Why?
Can be useful to implement full pagination / show total related
documents count in the UI.
### How?
Implements the logic in database adapters. In MongoDB it's additional
`$lookup` that has `$count` in the pipeline. In SQL, it's additional
subquery with `COUNT(*)`. Preserves the current behavior by default,
since counting introduces overhead.
Additionally, fixes a typescript generation error for join fields.
Before, `docs` and `hasNextPage` were marked as nullable, which is not
true, these fields cannot be `null`.
Additionally, fixes threading of `joinQuery` in
`transform/read/traverseFields` for group / tab fields recursive calls.
3436fb16ea