feat(x-aep-field): add field number (#48)

field numbers are not officially required by the AEPs, but are useful
for enumeration in intermediary representations such as resource
definitions for aep-lib-go.

Author: toumorokoshi

json_schema/extensions/x-aep-field.yaml

@@ -37,3 +37,9 @@ properties:
         "library.example.com/topic"
     type: array
     items: string
+  field_number:
+    description: |
+      A unique identifier for the field. This is useful when doing translation
+      to protocols that require indexing of fields, such as Protocol Buffers.
+    type: integer
+    minimum: 1