Allow custom prefix for cogroup field names on RHS

When there are multiple cogroups in a pipeline the prefix '__rhs__' can collide if the fields have a common name. This allows us to specify a custom prefix.

eg.
```java
Pump.cogroup(tableA, users, "user_id")
    .cogroup(tableB, new LeftJoin(), "user_id");
```
will conflict, but can instead be
```java
Pump.cogroup(tableA, "users_", users, "user_id")
    .cogroup(tableB, "tableB_", new LeftJoin(), "user_id");
```