Afform - Add default values to metadata

[colemanw]

Overview

Make explicit what were previously implied default values.
This is a bite-sized piece of #27783

Before

Less explicit metadata.

After

More explicit metadata.

Technical Details

Declaring defaults here requires less wrangling deeper-down in the Afform code, and also makes them discoverable to 3rd-party devs or anyone casually browsing the API Explorer.

[civibot]

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