Add config validation subcommand

As a user I want to ensure my config is setup correctly.
As an APS team dev I want to ensure my user's config set up correctly during troubleshooting or bug reports.

### Acceptance Criteria

Add a `gwa config validate` subcommand. This command will exit the program and print nothing if the application doesn't find any issues or potential warnings. An example output could be:

```shell
gwa config validate
[WARN] namespace not set
[ERROR] scheme set to ws, which is invalid. Must only be http or https.
```

Not required, but output flags of  `--json` could be added, which would write the output to a JSON file.

#### Rules

- `host`: required, must be a valid hostname
- `api_key`: warning if empty (some operations could be executed without it)
- `namespace`: warning if empty
- `scheme`: required, http and https only accepted values
- `token_endpoint`: required, must be valid URL

### Notes

Recommend merging #95 before starting work on this ticket, as that PR introduces a logging pattern that would be useful for this work.