Using regional sts endpoint by default

Currently, GRC uses a global sts endpoint(sts.amazonaws.com).
AWS CLI v2 and major sdk use regional sts endpoint by default.
We need to configure `sts_regional_endpoints = regional` for aws profiles when using vpc endpoints on isolated subnets even if using AWS CLI v2.

```
[profile cross-account]
role_arn = arn:aws:iam::1234567890:role/cross-account
source_profile = default
region = us-west-2
sts_regional_endpoints = regional
```

AWS STS Regionalized endpoints
https://docs.aws.amazon.com/sdkref/latest/guide/feature-sts-regionalized-endpoints.html
> Note: All new SDK major versions will default to regional in the future.

> AWS CLI v2 Partial Default value is regional.