Support pipeline-style updates in query builder

[alcaeus]

Feature Request

Q	A
New Feature	yes
RFC	no
BC Break	no

Summary

With MongoDB 4.2 and version 1.5 of the MongoDB library, it is possible to pass an aggregation pipeline to update operations (findAndModify, update, updates in bulk writes). This should be leveraged in the query builder by integrating a subset of aggregation features.

Possible syntax:

$queryBuilder
    ->field('field')
    ->equals('value')
    ->updateOne()
    ->pipeline()
        ->addFields() // or its alias $set
        ->project() // or its alias $unset
        ->replaceRoot() // or its alias $replaceWith
    ->end()

Other pipeline stages should not be available as they would create a query error.