Add get_by_fingerprint to SSHKeyClient

[LKaemmerling]

This PR adds a new method
client.ssh_keys.get_by_fingerprint("fingerprint")

Which allows to get an ssh key by its fingerprint. The fingerprint is unique.

Currently this is the solution we will replace with the new method:

            if self.module.params.get("fingerprint") is not None:
                result = self.client.ssh_keys.get_all(fingerprint=self.module.params.get("fingerprint"))
                if len(result) == 1:
                    self.hcloud_ssh_key = result[0]