Support inline role_arn for IAM Role authentication

See this issue for context : https://github.com/dbt-labs/dbt-redshift/issues/842

The connector need to be able to support IAM Role via inline parameters in addition to via an AWS profile, the same way it does for IAM Users.

If I open a connection with `role_arn`, `source_access_key_id`, and `source_secret_access_key`, currently it ignores `role_arn` and uses the access key to open an IAM user connection.
What I need instead is to leverage the access key to assume the role - see [boto3 credentials](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/credentials.html#assume-role-provider), we're in the [assume role provider chapter](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/credentials.html#assume-role-provider), particularly:
> If MFA authentication is not enabled then you only need to specify a role_arn and a source_profile.

What I need is to pass all the parameters inline, not a `source_profile`.

Empieza leyendo cómo el conector gestiona las credenciales IAM inline y la selección del perfil de AWS; después, compara el comportamiento solicitado con la documentación enlazada de boto3 assume-role provider y con el issue de contexto de dbt-redshift. Se considera terminado cuando role_arn se respeta junto con source_access_key_id y source_secret_access_key para asumir el rol, en lugar de abrir una conexión de usuario IAM.

Escrito por el modelo de indexación a partir del texto del issue.