Bugfix/cvd amends

[i-just]

Description

When a thumbable field is added to the layout designer, a new option is added to the “Thumbnail Source” dropdown. This PR:

• ensures that the text of that new option uses the field’s label, not handle
• prefixes that option’s value with layoutElement:; this ensures that the preview is rendered as expected straight away and that the selected thumb source is stored on the first save (as opposed to disappearing on first save and having to be selected and saved again)

Other tweaks:

• fixes an issue where the “Date Uploaded” attribute for Asset Volume was not using the parent placeholder value (a05b7d7)
• fixes an issue that could occur if the attribute didn’t specify a placeholder (5e42ee7)
• default field key should be layoutElement:<uid>; this is mostly used by the native fields; without this change, most of the native fields were not pulling in their preview HTML as expected; e.g. settings > addresses if you selected a Country field under Card Attributes or added a Lat/Long field and selected that one; (dbae397 and 514b345)
• cvd placeholder for address should be pulled from the native Address field (7ff5f7b)

Related issues

PR #18206