api: each API defines its 'endpoint' name

Add a new abstract class property to `BaseAPI` called `endpoint`,
meant to be implemented by deriving classes in order to identify
the end point name for the API provider.
Implement the new property in all existing API providers.

test/mod/test_api.py

@@ -17,6 +17,8 @@ class APITester(osbuild.api.BaseAPI):
         self.clean = False
         self.messages = 0
 
+    endpoint = "test-api"
+
     def _dispatch(self, server):
         msg, _, addr = server.recv()
         self.messages += 1