Extend field validation to also allow for arrays

Issue information

From user: jurgenhaas

Contribution record

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Problem/Motivation

When validating field values in \Drupal\modeler_api\Component::validate, the interface expects either a string or NULL. But there could also be arrays in some rare circumstances, like e.g. in the "Entity Diff" action plugin from ECA.

Proposed resolution

Implode arrays for the duration of validation and replace the array value back at the end.