Allow undefined fields in Options and HostOptions

Plugins seem to be using oscrc and osc.conf.config to store their config options.
All fields that are not known to osc are now stored in the 'extra_fields' dictionary
and handled in __getitem__() and __setitem__() as they were regular fields.
Such values are not checked for their types and the dictionary simply holds
strings obtained from oscrc or anything the plugins set through the python API.

tests/test_conf.py

@@ -78,6 +78,7 @@ maintained_update_project_attribute = OBS:UpdateProject
 show_download_progress = 0
 vc-cmd = /usr/lib/build/vc
 status_mtime_heuristic = 0
+plugin-option = plugin-general-option
 
 [https://api.opensuse.org]
 credentials_mgr_class=osc.credentials.PlaintextConfigFileCredentialsManager
@@ -97,6 +98,7 @@ trusted_prj = openSUSE:* SUSE:*
 downloadurl = http://example.com/
 sshkey = ~/.ssh/id_rsa.pub
 disable_hdrmd5_check = 0
+plugin-option = plugin-host-option
 """
 
 
@@ -401,6 +403,22 @@ class TestExampleConfig(unittest.TestCase):
         host_options = self.config["api_host_options"][self.config["apiurl"]]
         self.assertEqual(host_options["disable_hdrmd5_check"], False)
 
+    def test_extra_fields(self):
+        self.assertEqual(self.config["plugin-option"], "plugin-general-option")
+        self.assertEqual(self.config.extra_fields, {"plugin-option": "plugin-general-option"})
+
+        self.config["new-option"] = "value"
+        self.assertEqual(self.config["new-option"], "value")
+        self.assertEqual(self.config.extra_fields, {"plugin-option": "plugin-general-option", "new-option": "value"})
+
+        host_options = self.config["api_host_options"][self.config["apiurl"]]
+        self.assertEqual(host_options["plugin-option"], "plugin-host-option")
+        self.assertEqual(host_options.extra_fields, {"plugin-option": "plugin-host-option"})
+
+        host_options["new-option"] = "value"
+        self.assertEqual(host_options["new-option"], "value")
+        self.assertEqual(host_options.extra_fields, {"plugin-option": "plugin-host-option", "new-option": "value"})
+
 
 class TestFromParent(unittest.TestCase):
     def setUp(self):