Change HAS condition type to always use String in @whereConditions

[DrPowerSlam]

https://github.com/nuwave/lighthouse/issues/2723

• [x] Added or updated tests
• [x] Documented user facing changes
• [x] Updated CHANGELOG.md

Changes

Changes the condition type of HAS arguments in @whereConditions to always use String type, so the column type isn't inherited from the parent node.

Breaking changes

Technically this is a breaking change, since users that currently use the unupdated code will go from an Enum type to a String type. However, I would argue that it's almost impossible to correctly use the current functionality, since you need a perfect overlap between columns in a node and all its relations to make it work.