Bug 21978: Add middle_name to api specification

This patch adds middle_name to the accaeptable fields in API requests
and responses.

Test plan
1) Search for a user using the API
2) Confirm the API responds with a 200

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

api/v1/swagger/definitions/patron.yaml

@@ -19,6 +19,11 @@ properties:
       - string
       - "null"
     description: patron's first name
+  middle_name:
+    type:
+      - string
+      - "null"
+    description: patron's middle name
   title:
     type:
       - string