Fix(redshift): add missing db_user parameter for IAM auth

[synthpieter]

Problem

SQLMesh's Redshift connection configuration was missing the db_user parameter, which is required by the redshift_connector library when using IAM authentication.

When enabling iam (iam: True) in the connection config it throws, understandably so, an InterfaceError

redshift_connector.error.InterfaceError: Connection parameter db_user must be specified when using IAM authentication

However, it was not allowed to add this parameter to the config.

Error: Invalid 'redshift' connection config:
  Invalid field 'db_user':
    Extra inputs are not permitted

Verify your config.yaml and environment variables.

So iam authentication was not really useable.

Solution

Added the db_user field to the RedshiftConnectionConfig class in three places:

1. Documentation - Added parameter description in the docstring and docs
2. Field Declaration - Added db_user: t.Optional[str] = None to the class
3. Connection Kwargs - Added "db_user" to the _connection_kwargs_keys property
4. Tests - Added db_user to corresponding test.

Usage

With this fix, you can now configure Redshift IAM authentication in config.yaml:

gateways:
  redshift:
    connection:
      type: redshift
      database: your_database
      host: your-cluster.region.redshift.amazonaws.com
      port: 5439
      cluster_identifier: your-cluster
      iam: True
      db_user: your_db_username

Fixes #2146

Disclaimer

This is my first contribution to an open source project ever, so please let me know if this is the right way of doing things :)
I locally give the fix a spin and it seems to fix the problem!

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