Establish conventions for network resource primary keys in Nexus

In the resources added in #2645, we have defined a switch port settings resource:

CREATE TABLE omicron.public.switch_port_settings (
    id UUID PRIMARY KEY,
    name STRING(63) NOT NULL,
    description STRING(512) NOT NULL,
    time_created TIMESTAMPTZ NOT NULL,
    time_modified TIMESTAMPTZ NOT NULL,
    time_deleted TIMESTAMPTZ
);

The actual settings are in their own tables, with a composite primary key like so:

CREATE TABLE omicron.public.switch_port_settings_bgp_peer_config (
    port_settings_id UUID,
    bgp_announce_set_id UUID NOT NULL,
    bgp_config_id UUID NOT NULL,
    interface_name TEXT,
    addr INET,

    PRIMARY KEY (port_settings_id, interface_name, addr)
);

A question raised during code review was whether or not the settings should also have their own id column (UUID) as the primary key, instead of a composite primary key. This issue was created for discussing this and tracking subsequent work if necessary.

Start by reviewing the switch_port_settings and switch_port_settings_bgp_peer_config schema examples in the issue, then inspect the resources added in #2645. Determine whether network resource settings should use UUID primary keys or composite keys, and document the agreed convention plus any follow-up changes needed.