spec: can we make SchemaRecord support arbitrary schema fields?

[runspired]

A thought I had:

can we add another category of methods to the schema service:

registerKind(kind: KindFn)
kind(field: Field | { kind: string }): KindFn;

this would either augment the default schema-record behaviors or (if we determine the factoring can support it) change the implementation such that all default behaviors for field kinds are implemented via this mechanism at all, making schema-record fully amorphous