vaults should publish Director's governed params more conveniently

[Chris-Hibbert]

What is the Problem Being Solved?

When upgrading the VaultFactory, it's necessary to retrieve the governed parameters from the old version, since the paramManager is not durable, and resets. The parameters from the individual vaultManagers are available via call on the director's publicFacet, but the director's top-level governed params are only published via a notifier and to vstorage, which are more work to retrieve during upgrade.

E(directorPF).getGovernedParams({ collateralBrand });

Description of the Design

Add a method to the publicFacet.

Security Considerations

These parameters are all published already, so there's no security exposure in revealing them more conveniently.

Scaling Considerations

None

Test Plan

simple regression tests.

Upgrade Considerations

simplifies upgrade