Support for AWS China accounts

[ansromanov]

I'm trying to use aws-mfa-login with an AWS China account and get the following error:

aws-mfa-login --config ~/.aws-cn.yaml
#####
source: aws-cn
destination: aws-cn-mfa
#####
2023/10/17 12:52:36 operation error STS: GetCallerIdentity, https response error StatusCode: 403, RequestID: c1895db0-5866-4a35-a447-367de51b1d4f, api error InvalidClientTokenId: The security token included in the request is invalid.

The issue is rising because aws-mfa-login is trying to log in with the eu-central-1 account which is absent in AWS China.
The quick fix is to remove the hardcoded region from func (updater *CredUpdater) init() function:
https://github.com/signavio/aws-mfa-login/blob/v0.1.28/action/login.go#L43

I've checked this quick fix and it works pretty well for both aws and aws-cn accounts.

What else do you think should be taken into consideration?
As I see github.com/aws/aws-sdk-go-v2 handles different IAM partitions well and no code changes are needed here:

AWS - arn:aws:: ....
AWS China- arn:aws-cn:: ...