Gets the actions performed on the field's trigger events (AA: keystroke, format, validate, calculate), or null.
public PdfFieldAdditionalActions? AdditionalActions { get; }
ISO 32000-2 §12.7.4.1, Table 226, and §12.6.3, Table 199; the field's own entry (not inheritable), read through GetFieldAdditionalActions(CosObject?, CosReference?). A field merged with its widget shares the dictionary with the widget's annotation triggers (AdditionalActions).