✨(oidc) add django-oauth-toolkit w/ configuration

This allows to use `people` as an identity provider using
OIDC and local users.
This commit is partial, because it does not manage a way to
create "local" users and the login page is the admin one, which
can't be used for non staff users or login with email.

src/backend/mailbox_oauth2/serializers.py

@@ -0,0 +1,48 @@
+"""
+Serializers for the mailbox_oauth2 app.
+"""
+
+from django.contrib.auth import authenticate
+
+from rest_framework import serializers
+
+
+class LoginSerializer(serializers.Serializer):
+    """
+    Serializer for user login authentication.
+
+    Validates the email and password fields required for user authentication.
+    """
+
+    email = serializers.EmailField(
+        help_text="User's email address for authentication",
+        required=True,
+    )
+    password = serializers.CharField(
+        write_only=True,
+        help_text="User's password for authentication",
+        required=True,
+    )
+
+    def create(self, validated_data):
+        """Do not allow creating instances from this serializer."""
+        raise RuntimeError("LoginSerializer does not support create method")
+
+    def update(self, instance, validated_data):
+        """Do not allow updating instances from this serializer."""
+        raise RuntimeError("LoginSerializer does not support update method")
+
+    def validate(self, attrs):
+        """
+        Validate the email and password fields.
+        """
+        email = attrs.get("email")
+        password = attrs.get("password")
+
+        if not (email and password):
+            raise serializers.ValidationError('Must include "email" and "password"')
+
+        attrs["user"] = authenticate(
+            self.context["request"], email=email, password=password
+        )
+        return attrs