[Feature][Spike] Allow adapters to specify multiple namespaces

[colin-rogers-dbt]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Currently an adapter can supply only one namespace prefix for macros. This poses a challenge if we want to sever/deprecate inheritance relationships for adapters while maintaining backwards compatibility (i.e. dbt-redshift and dbt-postgres). Instead we'd like an adapter to be able to supply the deprecated parent adapters namespace along with it's own.

Relevant code section: https://github.com/dbt-labs/dbt-core/blob/main/core/dbt/context/providers.py#L143

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response

[github-actions]

This issue has been marked as Stale because it has been open for 180 days with no activity. If you would like the issue to remain open, please comment on the issue or else it will be closed in 7 days.

[github-actions]

Although we are closing this issue as stale, it's not gone forever. Issues can be reopened if there is renewed community interest. Just add a comment to notify the maintainers.