Track in the plan the types associated with PARAM_EXEC parameters.

Track in the plan the types associated with PARAM_EXEC parameters.

Up until now, we only tracked the number of parameters, which was
sufficient to allocate an array of Datums of the appropriate size,
but not sufficient to, for example, know how to serialize a Datum
stored in one of those slots.  An upcoming patch wants to do that,
so add this tracking to make it possible.

Patch by me, reviewed by Tom Lane and Amit Kapila.

Discussion: http://postgr.es/m/CA+TgmoYqpxDKn8koHdW8BEKk8FMUL0=e8m2Qe=M+r0UBjr3tuQ@mail.gmail.com