Set unique_field in credentials.py

[aranke]

Required in dbt 1.5+ to avoid NotImplementedError raised here: https://github.com/dbt-labs/dbt-core/blob/3cee9d16fa39012e30a02c66770d6b6c7a445f6c/core/dbt/contracts/connection.py#L141

[jwills]

@aranke thanks for this; I think the closest thing to a unique field for tracking purposes is either 1) remote.host + remote.port (if they're defined) or 2) path + external_root (if they're not.)

[aranke]

Updated, thanks!