Store the metadata v3 reads in the v2 catalog, from PolicyEngine.py's public records #4022

The app's newer workspace (v3) reads several metadata fields from /{country}/metadata that the v2 catalog does not store: each model's entities (key, plural name, whether it is the person entity) and basic inputs (the household builder's default fields); each variable's input and hidden-input flags, its module, the parameter that names what it adds or subtracts, and the labels of its enum values; each parameter's economy and household flags; and bracket schedules as nodes of the parameter tree with their labels.

Without these fields, v3 cannot load its metadata from the v2 routes. PolicyEngine.py now publishes them in its public records (PolicyEngine/policyengine.py#701), so the catalog can take them from there instead of from the country package's tax-benefit system or a private PolicyEngine.py method.

What is needed:

• Store the fields when a catalog version is published (catalog format 2), read only from PolicyEngine.py's public records.
• Keep catalog versions published earlier unchanged: their new fields stay empty.
• Check each new field against what /{country}/metadata publishes for the US and the UK.
• Move the Public API's PolicyEngine.py dependency to the release that publishes the fields.