Rename ssh methods

To make this client more in line with the [official API](https://gridscale.io/en/api-documentation/index.html#tag/SSH-key) I'd to propose a rename of the following methods:

- get_ssh_keys
- get_ssh_key
- create_ssh_key
- update_ssh_key
- delete_ssh_key
- get_ssh_key_events

`ssh_key` is canonically without the underscore in the API. The new function names would then be

- get_sshkeys
- get_sshkey
- create_sshkey
- update_sshkey
- delete_sshkey
- get_sshkey_events

For backwards-compat we could alias the methods to their old names.

It's a small annoyance, but I still wanted to mention it. I'm ready to work an a pull request if this change finds acceptance.