APIv4 - Fix getFields to respect default_value from getFields

[colemanw]

Overview

A few fixes to make APIv4 getFields more internally consistent.

Technical Details

GetFields had an odd way of setting defaults for field metadata; this reuses the default_value property instead of declaring defaults in an ad-hoc fashion.

Also adds readonly to the table_name CustomGroup column, since that cannot be changed by the user.

[civibot]

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