Unflatten exposures in migration

[alanmcruickshank]

Currently we wipe the "depends_on_nodes" field. This updates the migration macro to be able to populate the field.

[alanmcruickshank]

I've run a very similar script (pasted below but less tidy than the version I've uploaded here) on our project and it works well:

...
select
    command_invocation_id,
    array_agg(output_feeds),
    null,
    maturity,
    name,
    node_id,
    null,
    package_name,
    null,
    type,
    null,
    artifact_generated_at
from tails_db.analytics_mart_data_ops.dim_dbt__exposures
group by command_invocation_id, maturity, name, node_id, package_name, type, artifact_generated_at

[NiallRees]

fyi @tacastillo