Configuring endpoint for "aws support"

Hi! aws-shell looks great!

I have another region as my default region, but "aws support" only has an endpoint in `us-east-1`. So I have to run it with `--region us-east-1` or else it doesn't work:

```
[~] $ aws-shell
aws> support describe-cases

Could not connect to the endpoint URL: "https://support.[redacted].amazonaws.com/"
aws>
```

It would be nice if I could configure aws-shell to use the sole available end-point for the service without changing my default region.

Best,
Aleksey

Start by tracing how aws-shell handles the configured default region and constructs endpoints for the `support describe-cases` command. Verify the command with a non-us-east-1 default region; done means AWS Support uses its available us-east-1 endpoint without requiring a per-command region override.

Written by the indexing model from the issue text.