Add health check

I've apparently got invalid creds and am getting
```
requests.exceptions.HTTPError: 401 Client Error: Unauthorized for url: https://api.earthdaily.com
```
Ideally we would have a method to check credentials on
```python
# Initialize client
config = EDSConfig()
client = EDSClient(config)
```
It could raise a warning if the EDS_API_URL and EDS_AUTH_URL are correct but either of EDS_CLIENT_ID or secret are not

Start at EDSClient(config) and trace the existing authentication request and error handling. Define how the health check should distinguish incorrect URLs from invalid client credentials, then add coverage for the reported 401 case and verify the resulting warning behavior.

Written by the indexing model from the issue text.