add proxy gateways with --px arg

Proxy gateways do not run workers, and are meant to be passed with the
`via` attribute to additional gateways.
They are useful for running multiple workers on remote machines.

Example usage:
```
pytest -sv --dist=load --px "id=my_proxy//socket=IP:PORT" --tx "5*popen//via=my_proxy"
```

Proxy gateways do not run workers, anda re meant to be passed

testing/test_plugin.py

@@ -295,7 +295,7 @@ class TestDistOptions:
     def test_getxspecs(self, pytester: pytest.Pytester) -> None:
         config = pytester.parseconfigure("--tx=popen", "--tx", "ssh=xyz")
         nodemanager = NodeManager(config)
-        xspecs = nodemanager._getxspecs()
+        xspecs = nodemanager._gettxspecs()
         assert len(xspecs) == 2
         print(xspecs)
         assert xspecs[0].popen
@@ -303,7 +303,7 @@ class TestDistOptions:
 
     def test_xspecs_multiplied(self, pytester: pytest.Pytester) -> None:
         config = pytester.parseconfigure("--tx=3*popen")
-        xspecs = NodeManager(config)._getxspecs()
+        xspecs = NodeManager(config)._gettxspecs()
         assert len(xspecs) == 3
         assert xspecs[1].popen