ProxyJump is not treated as a list

[aiven-amartin]

Hi,

In https://github.com/ronf/asyncssh/blob/develop/asyncssh/connection.py#L6996, ProxyJump is treated as if it can only contain a single hostname, but per the spec it is actually a space separated list of hosts to tunnel through. See e.g. https://man.openbsd.org/ssh_config#ProxyJump.

A test case like the below should be sufficient to validate a solution, though setting up additional servers for the test may be prudent.

    @asynctest
    async def test_proxy_jump(self):
        """Test connecting a tunnneled SSH connection using ProxyJump"""

        write_file('.ssh/config', 'Host target\n'
                   '  Hostname localhost\n'
                   f'  Port {self._server_port}\n'
                   f'  ProxyJump localhost:{self._server_port} localhost:{self._server_port}\n'
                   'IdentityFile ckey\n', 'w')

        try:
            async with self.connect(host='target', username='ckey'):
                pass
        finally:
            os.remove('.ssh/config')

I will try to submit a draft PR with a potential fix shortly.