Fixes relation reference widget when chain filter option is activated, fixes #16903 (backport)

[pblottiere]

Description

Backport of #4904 and the follow up #4932 to fix an issue regarding the relation reference widget : https://issues.qgis.org/issues/16903

When the Chain Filter option is activated, a filtering is now resolved on the fly with cached values in order to provide only filtering values (in combo boxes) leading to actual features.

Some tests have been added to validate the behavior.

Checklist

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