support parameterized (LATERAL) foreign joins

Hi hackers,

create_foreign_join_path() currently rejects any parameterized foreign
join outright:

```
if (!bms_is_empty(required_outer) || !bms_is_empty(rel->lateral_relids))
elog(ERROR, "parameterized foreign joins are not supported yet");
```

and postgres_fdw's postgresGetForeignJoinPaths() side-steps that by
declining to push down any join whose relation has lateral references:

```
/* This code does not work for joins with lateral references ... */
if (!bms_is_empty(joinrel->lateral_relids))
return;
```

For example in the below query (foreign i1/i2, local o):

SELECT o.f1, ss.x
FROM local_tbl o,
LATERAL (SELECT 1 AS x
FROM (SELECT o.f1 AS lat, i2.f1 AS loc
FROM int8_tbl i1, int4_tbl i2) ss1
RIGHT JOIN int4_tbl i3 ON (i3.f1 > 1)
WHERE ss1.loc = ss1.lat) ss;

The RIGHT JOIN here keeps the LATERAL reference from being flattened into
an ordinary join clause, so the inner foreign join (i1 x i2) really does
require a parameterized path and is never pushed down.

The attached patch teaches the planner to build such paths and as an
example to postgres_fdw.

create_foreign_join_path() no longer errors on a parameterized request.
Instead it builds a ParamPathInfo via a new helper,

get_joinrel_parampathinfo_pushdown(joinrel, required_outer,
restrict_clauses, rows)

in relnode.c. The FDW does not have a pair of input paths
to describe how the join is formed, so instead it hands us the
parameterization, the estimated rowcount, and the RestrictInfos the path
will enforce.