Auto create identiy key when post a json to a non-exist table.

If header has 'x-pgrest-create-identity-key: yes'
then add _id field to target table .

Example:

$ curl -X POST \
  -H "Content-Type: application/json" \
  -H "x-pgrest-create-identity-key: yes"
  -d @data.json http://127.0.0.1:3000/collections/data