#3570354 - Let users configure warning messages

lysenkorequested to merge
issue/unique_field-3570354:3570354-let-users-configure into 8.x-2.x
  1. Added settings form
  2. Updated config schema of the module
  3. Made settings for the warning message translatable
  4. Updated validation callback in order to provide ability to use the message from the settings form added default validation message.

Related to #3570354

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