Add EstimateCost() API to FdwRoutine, which is used to estimate

Add EstimateCost() API to FdwRoutine, which is used to estimate
the costs of a ForeignScan.

Also implement pgEstimateCost() for postgresql_fdw, which estimate
the costs as sum of:
  * costs of sequencial scan
  * connection cost (if connection_cost was set in the server generic
    option)
  * transfer cost (if transfer_cost, cost-per-width, was set in the
    server generic option)