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`.

Comece lendo como o connector trata credenciais IAM inline e a seleção do perfil da AWS; em seguida, compare o comportamento solicitado com a documentação vinculada do boto3 assume-role provider e com a context issue do dbt-redshift. Considera-se concluído quando role_arn é respeitado com source_access_key_id e source_secret_access_key para assumir a role, em vez de abrir uma conexão de usuário IAM.

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