Enable datasources to contribute policy statements

It is useful for a datasource to be able to contribute policy statements in at least 2 situations.

(1) the datasource driver knows of some convenient helper tables that can be defined on top of its explicit tables and should therefore be able to insert them into its policy. The presence of these rules only serves to extend the tables provided by the datasource; it does not require the policy writer to use them or even know about them.

(2) The datasource driver has action-descriptions that define the semantics of actions used in the simulate call. It's nice to make the datasource driver the source of truth for these actions, since then all the datasource-specific information is kept in one place.

As part of the datasource driver installation, Congress should query the datasource driver to ask about any rules that it wants to contribute and store them within the policy for that datasource driver. (This will require relaxing some of the safety checks in the nonrecursive-rule-theory to differentiate action-description rules from regular rules.)