fix: Allow non-aws partitions to be used, including GovCloud

[suprememoocow]

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Currently, this module assumes this will always be run in the aws partition. This means that if this is applied in GovCloud, it fails with an invalid partition.

This change uses the current partition, rather than hardcoding to aws. For GovCloud, it will use aws-us-gov.

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[suprememoocow]

@ivankatliarchuk good day! I'm hitting a problem when trying to use this module in AWS GovCloud.

I get the following error: Partition "aws" is not valid for resource.

╷
│ Error: creating IAM Policy (EnforceMFA): operation error IAM: CreatePolicy, https response error StatusCode: 400, RequestID: XXXX, MalformedPolicyDocument: Partition "aws" is not valid for resource "arn:aws:iam::XXXX:mfa/${aws:username}".
│ 
│   with module.aws.module.user_group_membership.module.enforce_mfa[0].aws_iam_policy.this,
│   on .terraform/modules/aws.user_group_membership.enforce_mfa/main.tf line 1, in resource "aws_iam_policy" "this":
│    1: resource "aws_iam_policy" "this" {
│ 
╵

This is because GovCloud does not use arn:aws but rather arn:aws-us-gov. Likewise, AWS China uses a different partition scheme too.

Would you mind reviewing this change please?