Implement user show auth-info command (ecctl)

@karencfv commented on [Thu Oct 03 2019](https://github.com/elastic/cloud-cli/issues/1014)

## Overview
Now that the swagger spec in `cloud-sdk-go` has been updated to `2.4` there are new endpoints we need to implement.

endpoint ID:
```
get-authentication-info
```
## Possible Implementation
New command to be implemented:

```console
$ ecctl user show auth-info
```

## Testing
All new functions should have their respective unit tests.

---

@karencfv commented on [Thu Oct 03 2019](https://github.com/elastic/cloud-cli/issues/1014#issuecomment-537733672)

~Blocked by: https://github.com/elastic/cloud-cli/issues/1013~