[diesel][cleanup] Make use of Diesel "Selectable" to cut down on repetition

[smklein]

(Follow-up from #192)

"model.rs" contains many structs which are used to map to Diesel DB tables, but within this file, there exists some
repetition.

As a prime example, "IdentityMetadata" is flattened and repeated for all tables that contain this similar-looking prefix.
Notably: The representation in the database is unique (there are some identity-related columns that happen to share the same name/type, but are separate in each table) but their in-memory representation is the same.

It would be nice to use a shared structure in Rust to represent these types.

Some of Diesel's derive macros support a field attribute #[diesel(embed)] which allows nested structures.
These include:

• Insertable
• Selectable

In contrast, the Queryable derive macro does not support this embedding - it requires that the count and order of the Queryable struct will match the on-disk format, column-for-column.

To make use of embedded structures (like this shared identity prefix), we'd need to do the following:

• For all structs which derive Queryable, also derive Selectable
• Derive Selectable on the structure we'd like to embed
• Replace the long-form column fields with the embed-ed structure
• Invoke .select(MyStruct::as_select()) when doing lookups of the nested object

There are a couple spots where this might be tricky:

• Updating the CTE to make this viable across generic types
• Allowing a "single structure" (like IdentityMetadata) to be used across multiple tables. This might require some macro wrapping - the Selectable derive macro requires a single table selection at declaration time.

[smklein]

This has been started a bit by https://github.com/oxidecomputer/omicron/pull/208/files , where I gave this a shot with DiskRuntimeState.