qe: support `_count` in joined queries (#4678)
Implement relation aggregations support for the `join` strategy:
* Build relation aggregation queries in the new query builder
* Add new methods to the abstractions that represent selections to account for
differences in representation in queries that use or don't use JSON objects
* Implement coercion and serialization logic
Next step: https://github.com/prisma/team-orm/issues/903
Part of: https://github.com/prisma/team-orm/issues/700
Closes: https://github.com/prisma/team-orm/issues/902
