Enable support for roles via the `entity` property in identifiers

There are many cases where identifiers represent entities in the world, but those entities can play different roles. For example, you might have a marketplace where users can be either buyers or sellers, or a lodging service where users can be guests or hosts.

The config spec for supporting this is to have each identifier `name` represent the role, and an optional `entity` value represent the specific entity. If no `entity` value is provided, the `name` represents both role and identity. We have test configs with a [basic user account entity setup](https://github.com/transform-data/metricflow/blob/main/metricflow/test/fixtures/model_yamls/simple_model/data_sources/users_latest.yaml#L19-L22) and a [corresponding entity/role mapping in a separate data source](https://github.com/transform-data/metricflow/blob/main/metricflow/test/fixtures/model_yamls/simple_model/data_sources/bookings_source.yaml#L60-L67).

The linked specifications theoretically allow you to write a query like:

```
mf query --metrics bookings --dimensions guest__home_state_latest
```

That should resolve the role of `guest` (based on the identifier name) to the entity `user` (based on entity value). So the underlying SQL would be the equivalent of:

```
SELECT b.guest__home_state_latest, a.bookings
FROM (
SELECT guest_id AS guest, SUM(1) AS bookings
FROM bookings_source
GROUP BY 1
) a
LEFT OUTER JOIN users_latest b
-- role value joined to corresponding entity value
ON a.guest = b.user
```

Today it throws an exception because the logic to process this resolution has not yet been implemented, and we insist on joining `ON a.guest = b.guest` but there is no `guest` entity in the model.