feat: add span resolver on DatasetExample gql type (#3394)

- Adds `span` resolver on `DatasetExample` type returning optional `Span`.
- Makes `span_rowid` a nullable foreign key with an index on the `dataset_examples` table.
- Defines a `sqlalchemy` relationship between dataset examples and the span the example came from.
- Adds corresponding tests.
- Optimizes a few pre-existing queries using `load_only`.