"lookup_resource" macro should support different primary keys

[davepacheco]

See #845. We want the new lookup API to support a few more cases:

• where there's a composite primary key (e.g., for built-in roles and update artifacts) and "id" is usually not present
• where there's no "name" column (e.g., for sleds, silo users)

[davepacheco]

This winds up being kind of a blocker for ssh key support (see #849) because SiloUser does not have a name column and it doesn't really make sense for it to have one. (The closest thing would be "external id". Even if we add this, it should have a more specific name than "name" (to distinguish it from "username", "first name", "surname", etc., and so that people realize it's not the same kind of "name" that all the other resources have) and it would definitely have a different type (since "Name" is highly specific to the requirements in RFD 4).)