feat: Add multi-select dropdown support for Forms questions

[RPGMais]

Summary

Fields plugin dropdowns configured with multiple=1 now work correctly when used in GLPI Forms through the PluginFieldsQuestionType ("Campo" question type).

Problem: When a Fields dropdown with multi-selection enabled was added to a Form, there was no way to enable multi-select behavior in the form editor. The PluginFieldsQuestionType lacked:

• A toggle to enable/disable multi-selection (like the native QuestionTypeDropdown has)
• Proper answer handling for array values in prepareEndUserAnswer()
• Correct serialization when applying multi-select answers to tickets via PluginFieldsDestinationField
• Multi-select condition handlers for visibility rules

Solution (3 files, 115 lines added):

• questiontypeextradataconfig.class.php -- Added is_multiple field to the serialized config (default false, fully backward-compatible)
• questiontype.class.php -- Added renderAdministrationOptionsTemplate() with "Allow multiple options" toggle, overrode prepareEndUserAnswer() to normalize multi-select arrays, updated renderEndUserTemplate() and renderAdministrationTemplate() to respect the flag, added MultipleChoiceFromValuesConditionHandler for multi-select fields
• destinationfield.class.php -- Handle both dropdown and dropdown-X types with str_starts_with(), json_encode array values for multi-select fields when creating tickets

How it works

1. In the form editor, select question type "Campo" (Field)
2. Choose the block and dropdown field
3. Toggle "Allow multiple options" on
4. Save -- the end-user form renders the dropdown as multi-select (Select2 tags)
5. Submitted answers are properly stored and applied to tickets

Design decisions

• The toggle is independent of the field's multiple DB flag, giving form editors flexibility to enable/disable multi-select per question regardless of the field definition
• isMultipleForQuestion() checks both the form config (extra_data.is_multiple) and the field definition (field.multiple) as fallback
• Follows the same pattern as the core QuestionTypeDropdown implementation (same toggle markup, same condition handler class)

No breaking changes

• is_multiple defaults to false in jsonDeserialize(), so existing questions are unaffected
• The destinationfield.class.php change also fixes the elseif to use str_starts_with('dropdown') instead of exact match == 'dropdown', correctly handling dropdown-X types (e.g., dropdown-User)

Test plan

• Create a Fields dropdown with multiple=1 in a block assigned to Tickets
• Add a "Campo" question to a Form, select the dropdown field, enable "Allow multiple options"
• Save the form -- verify extra_data contains is_multiple: 1
• Render the form as end-user -- verify the dropdown shows as multi-select (Select2 tags)
• Select multiple values -- verify tags appear correctly
• Submit the form -- verify the answer is stored as a JSON array
• Verify the created ticket has the correct multi-select values in the Fields container
• Verify condition/visibility rules work with multi-select answers
• Verify backward compatibility: existing "Campo" questions without is_multiple still work as single-select