Add possibility to save multiple values as JSON

[fastslack]

Summary of Changes

This change add the possibility to save multiple values of FieldList as JSON.

Testing Instructions

Create using JForm a list with multiple value enabled like:

    <field multiple="true" name="options" type="list" label="COM_TEST_FORM_LBL_OPTIONS" description="COM_TEST_FORM_DESC_OPTIONS" hint="COM_TEST_FORM_LBL_OPTIONS">
      <option value="0">COM_TEST_PRODUCTS_OPTIONS_0</option>
      <option value="1">COM_TEST_PRODUCTS_OPTIONS_1</option>
      <option value="2">COM_TEST_PRODUCTS_OPTIONS_2</option>
    </field>

Then add into save() function in models:

    $data['options'] = json_encode($data['options']);

Expected result

You can save multiple values as JSON into database

[laoneo]

Sorry that it took so long to respond. While this is a nice addition for Joomla, it should be added to version 4 as it is a new feature. Can you rebase it to the 4.2-dev branch? In the meantime I'm closing the pr, when ready please reopen so we can properly test it. Thanks for your contribution, making Joomla better.