Add sparse fieldset support

[dresslerdemos]

Sparse fieldsets in the response are already supported via Fractal. However, fetching entities via DQL may profit from sparse fieldset considerations too.

• Maybe possible using partial objects: https://stackoverflow.com/questions/4923817/what-is-a-proxy-in-doctrine-2/17787070#17787070
• Partial objects are deprecated, but there are alternatives, see https://www.doctrine-project.org/projects/doctrine-orm/en/2.9/reference/partial-objects.html#partial-objects and Deprecate Partial Objects doctrine/orm#8471