Issue #3530779 by detroz, sriram s: Add config schema for the focal_point_crop_by_width effect

Sriramrequested to merge
issue/focal_point-3530779:3530779-schema-missing-for into 2.x

Adds the missing image.effect.focal_point_crop_by_width config schema entry (mirrors the existing focal_point_crop / focal_point_scale_and_crop entries), fixing SchemaIncompleteException on recipe install / config validation. Carries detroz's patch forward and adds a Kernel regression test.

Closes #3530779

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