Use SQL composite data types for mutli-parent resource

[luqmana]

We have some tables with columns being foreign keys to potentially multiple tables depending on some kind or type, e.g.:

CREATE TYPE omicron.public.network_interface_kind AS ENUM (
    /* An interface attached to a guest instance. */
    'instance',
    /* An interface attached to a service. */
    'service'
);

CREATE TABLE omicron.public.network_interface (
    /* ...snip... */

    /* The kind of network interface, e.g., instance */
    kind omicron.public.network_interface_kind NOT NULL,

    /*
     * FK into the parent resource of this interface (e.g. Instance, Service)
     * as determined by the `kind`.
     */
    parent_id UUID NOT NULL,

Keeping track of 2 separate columns can be kind of fraught so it'd be nice to replace with a composite data type on the SQL side and an enum + appropriate impls on the Rust side.