Move initialized action callable to private field

[grahamalama]

When we included the callable in a public model field, we ran into a bug (#136 and #137) where jsonable_encoder couldn't serialize an action with an initialized requests.Session object.

Here's how I recreated the error we saw:

diff --git a/tests/unit/jbi/bugzilla_action.py b/tests/unit/jbi/bugzilla_action.py
new file mode 100644
index 0000000..d91e324
--- /dev/null
+++ b/tests/unit/jbi/bugzilla_action.py
@@ -0,0 +1,16 @@
+from bugzilla import Bugzilla
+from requests import Session
+
+session = Session()
+
+
+class TestBugzillaAction:
+    def __init__(self):
+        self.bz = Bugzilla(url=None, requests_session=session)
+
+    def __call__(self):
+        return lambda: "test"
+
+
+def init():
+    return TestBugzillaAction()
diff --git a/tests/unit/test_models.py b/tests/unit/test_models.py
new file mode 100644
index 0000000..6dc37ca
--- /dev/null
+++ b/tests/unit/test_models.py
@@ -0,0 +1,15 @@
+from src.jbi.models import Action
+from fastapi.encoders import jsonable_encoder
+
+
+def test_model_serializes():
+    action = Action.parse_obj(
+        {
+            "whiteboard_tag": "devtest",
+            "contact": "[email protected]",
+            "description": "test config",
+            "module": "tests.unit.jbi.bugzilla_action",
+        }
+    )
+    assert callable(action.callable)
+    jsonable_encoder(action)

For some reason, excluding the callable field with Config didn't seem to fix the bug. So instead, in this PR we move the callable to a private field with Pydantic's PrivateAttr.

We also add a test case as a regression test for the bug we observed.

This seems like more of a bug with jsonable_encoder, but it's probably good to remove the callable from the dict representation of the model anyway.