Include REFERENCE_TABLE and make PropertyFormItemEditConfig abstract

[dnlkoch]

Description

This adds the component type REFERENCE_TABLE to the available feature attributes fields. It can be used to configure a 1:n relationship of the given field. Example:

{
  "propertyName": "nutzungen",
  "displayName": "Nutzungen",
  "component": "REFERENCE_TABLE",
  "tablePropertyName": "Beschreibung",
  "editFormConfig": [
    {
      "propertyName": "id",
      "displayName": "ID",
      "component": "NUMBER",
      "readOnly": true
    },
    {
      "propertyName": "Beschreibung",
      "displayName": "Beschreibung",
      "component": "TEXTAREA",
      "required": true
    }
  ]
}

Please review @terrestris/devs.

Related issues or pull requests

• fix: determine discriminator values shogun-admin#165
• feat: include REFERENCE_TABLE type shogun-util#384
• Allow configurable (1:n) JSON fields in FeatureInfo and Edit forms shogun-gis-client#1164

Pull request type

• Bugfix
• Feature
• Dependency updates
• Tests
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe)

Do you introduce a breaking change?

• Yes
• No

Checklist

• I understand and agree that the changes in this PR will be licensed under the
  Apache Licence Version 2.0.
• I have followed the guidelines for contributing.
• The proposed change fits to the content of the code of conduct.
• I have added or updated tests and documentation, and the test suite passes (run mvn test locally).
• I have added a screenshot/screencast to illustrate the visual output of my update.

[dnlkoch]

Thanks for the review!