feat(versioning): SkipUnmodifiedPlugin audit-key normalize for Dashboard.json_metadata

Continuum's no-op suppression compared post-flush column values
byte-for-byte against the previous live shadow row. For
``Dashboard.json_metadata`` that produced false-positive version rows
on saves where the user authored nothing — the frontend re-stamps
``map_label_colors`` (regenerated from the ``LabelsColorMap``
singleton) on every save, plus ``chart_configuration`` /
``global_chart_configuration`` / ``show_chart_timestamps`` /
``color_namespace`` (derived from the current chart set), so two
consecutive identical saves produce different bytes for the column.
The diff engine already excluded those keys via
``DASHBOARD_JSON_METADATA_AUDIT_KEYS`` when computing change records;
the skip-plugin diverged.

Adds a ``_COLUMN_NORMALIZERS`` registry keyed on
``(class_name, column_name)`` that maps to a per-column normalizer
applied to both pre- and post-image before equating. The first
entry parses ``Dashboard.json_metadata`` as JSON and drops the
audit-key set before comparing. The same registry is the extension
point for analogous transient fields on charts and datasets.

Promotes ``_DASHBOARD_JSON_METADATA_AUDIT_KEYS`` to a public name
(``DASHBOARD_JSON_METADATA_AUDIT_KEYS``) so the skip-plugin can import
it from ``superset.versioning.diff`` without reaching across a
leading-underscore boundary.

Integration coverage: ``test_map_label_colors_only_change_does_not_create_version``.