osbuild: auto-generate socket addresses for APIs

Rely on the ability of `BaseAPI` to auto-generate socket addresses
when no one was provided. The `BuildRoot` does not rely on the
sockets being created in the `BuildRoot.api` directory anymore and
will instead bind-mount each individual socket address to the well
known location via the `BaseAPI.endpoint` identifier.
Convert all API providers to take the `socket_address` as an
optional keyword argument.

osbuild/api.py

@@ -105,7 +105,7 @@ class API(BaseAPI):
 
     endpoint = "osbuild"
 
-    def __init__(self, socket_address, args, monitor):
+    def __init__(self, args, monitor, *, socket_address=None):
         super().__init__(socket_address)
         self.input = args
         self._output_data = io.StringIO()