feat: add support for Redshift connections

[owenjonesuob]

An attempt to add support for Amazon Redshift connections.

Redshift is based on Postgres - which is great, because {dm} already has excellent Postgres support 😁

So for the most part, we can just borrow existing functionality.

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There are some differences between Redshift and Postgres - more detains in the AWS docs. One note in particular caught my eye:

Unique, primary key, and foreign key constraints are permitted, but they are informational only. They are not enforced by the system, but they are used by the query planner. - https://docs.aws.amazon.com/redshift/latest/dg/c_unsupported-postgresql-features.html

I'm not sure if this has any impact on whether we can build a dm by learning the database keys.

I also tried, but failed, to write a Redshift-friendly version of postgres_column_constraints since Redshift doesn't support some of the syntax used there (e.g. array_position()). This affects dm_meta_raw() - for now I have added an implementation based on the final else clause.

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Testing any Redshift functionality directly might be tricky, since it would require a Redshift warehouse in an AWS organisation - i.e. a lot of additional infrastructure (and cost?!). Would we be happy to rely on the fact that it's "nearly Postgres", and therefore mostly covered by Postgres testing? Or can we think of some way to test directly?

Edit: that is, testing via CI/CD - otherwise we are perhaps relying on ad hoc testing by users who happen to have access to a Redshift warehouse...

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Requested indirectly in #1678.

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[krlmlr]

Thanks! I have no easy way to test this, I trust this works for you.