Implement protobuf_each()

[rgov]

Ideally this would be a table-valued function that is similar to
json_each() from the JSON1 extension:

SELECT protobuf_extract(protobuf, "Person", "$.name")
  FROM people, protobuf_each(people, "Person", "$.phones")
 WHERE protobuf_each.value LIKE "607-%";

Without this function, there is no way to loop over elements of a repeated field.