Add region support when calling GetClusterCredentials

[bendrucker]

When calling GetClusterCredentials, users must supply the region where the Redshift cluster is located. Currently, there is no explicit provider configuration field for this input. As noted in #58 (comment), you can use AWS_REGION to set this.

I checked the AWS provider and mirrored its simple field implementation, with no validation:

https://github.com/hashicorp/terraform-provider-aws/blob/19469514cc16736d80ab976aa3178d5aaedbeba8/internal/provider/provider.go#L292-L297

[bendrucker]

Hi @winglot @pawelaugustyn, wondering if you're planning to actively maintain this provider. It's been a big help for @TakeScoop and I'm happy to continue submitting improvements if it's of interest. Otherwise, happy to fork if you're not looking for outside contributions.

[mtesch-um]

I'll second that - it's been a huge help for us too - we're more than happy to help out with maintenance if there's anything we can do.

[winglot]

@bendrucker Sorry for the delays. I had a busy couple of weeks. I have the review and merge processes on my agenda for the beginning of next week.

[bendrucker]

Quite alright, thanks for the update!