perf: Allow covering relation queries with minimal index (#6581)

* Apply linter changes on files I'm about to update

My actual changes were quite difficult to find when buried in this sea
of style changes, which were getting automatically applied during a
pre-commit hook. Here I just run the hooks against the files I'm going
to be touching in the following commit, so that a reviewer can ignore
these automatically generated diffs and just view the meaningful commit.

* perf: Allow covering relation queries with minimal index

When finding objects through a relation, we're sending Mongo queries
that look like this:
```
db.getCollection('_Join:foo:bar').find({ relatedId: { $in: [...] } });
```

From the result of that query, we're only reading the `owningId` field,
so we can start by adding it as a projection:
```
db.getCollection('_Join:foo:bar')
  .find({ relatedId: { $in: [...] } })
  .project({ owningId: 1 });
```

This seems like the perfect example of a query that could be satisfied
with an index scan: we are querying on one field, and only need one
field from the matching document.

For example, this can allow users to speed up the fetching of user roles
in authentication, because they query a `roles` relation on the `_Role`
collection. To add a covering index on that, you could now add an index
like the following:
```
db.getCollection('_Join:roles:_Role').createIndex(
  { relatedId: 1, owningId: 1 },
  { background: true }
);
```

One caveat there is that the index I propose above doesn't include the
`_id` column. For the query in question, we don't actually care about
the ID of the row in the join table, just the `owningId` field, so we
can avoid some overhead of putting the `_id` column into the index if we
can also drop it from the projection. This requires adding a small
special case to the MongoStorageAdapter, because the `_id` field is
special: you have to opt-out of using it by projecting `{ _id: 0 }`.