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`.

首先阅读 connector 对内联 IAM 凭证和 AWS 配置文件选择的处理方式，然后将所请求的行为与链接的 boto3 assume-role provider 文档以及 dbt-redshift 上下文 issue 进行比较。完成标准是：使用 role_arn 以及 source_access_key_id 和 source_secret_access_key 来承担该角色，而不是打开 IAM 用户连接。

由索引模型根据 Issue 内容生成。