Missing "external" relation type

[izeigerman]

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Describe the bug

The Databricks adapter is missing the external relation type which is a part of the dbt adapter contract: https://github.com/dbt-labs/dbt-adapters/blob/main/dbt/adapters/contracts/relation.py#L23. This type is usually assigned to relations associated with sources.

Steps To Reproduce

Currently the issue surfaces when trying to load the dbt project with SQLMesh (https://github.com/tobikodata/sqlmesh/) with the adapter version > 1.8.0. On version 1.8.0 and below it works as expected. I imagine it may impact other areas as well since the dbt relation type clearly expects this type to exist: https://github.com/dbt-labs/dbt-adapters/blob/main/dbt/adapters/base/relation.py#L367-L368

[izeigerman]

For reference here's the external relation type in the snowflake adapter: https://github.com/dbt-labs/dbt-snowflake/blob/main/dbt/adapters/snowflake/relation_configs/policies.py#L11