Add function getUserBySSHKeyId in keysAPIS.java

[joe-flumen]

In keysAPIS.java

Add API to get key by keyid, as from gitlab api docs

    /**
     * @param id The ID of the SSH key
     * @return The Key which includes the user who owns the key
     * @throws GitLabApiException If anything goes wrong
     */
    public Key getUserBySSHKeyId(Long id) throws GitLabApiException {
        Response response = get(Response.Status.OK, null, "keys", id);
        return response.readEntity(Key.class);
    }