Use exposed filters to allow query string

Problem/Motivation

The GeoJSON display type allows to configure exposed filters, but they are not actually used since the class overrides the DisplayPluginInterface::usesExposed method and unconditionally returns FALSE.

Proposed resolution

Use exposed filters, as the REST base plugin does, to allow query string.

Remaining tasks

Review.

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

TBD.