Support for application credentials

[mxmxchere]

As far as i can see (https://github.com/stackitcloud/yawol/blob/main/internal/openstack/client.go#L209) yawol currently only supports username and password to authenticate against openstack. Is that correct? If so, can you add support for application credentials as well?

[dergeberl]

Thanks for your proposal 🎉 .

I think we never tested it with application credentials and I also dont see a implementaion for it.
We need to check when we have time to take a look into this.

Feel free to also take a look and open an PR for application credentials.