Add wait stats

[Alena0704]

Report cross-slice ShareInputScan wait

A ShareInputScan consumer placed in a slice other than its producer blocks
until the producer publishes the shared tuplestore. The wait is charged to
the node's own time, so an idle node looks slow, and a query killed by
statement_timeout while blocked leaves no trace at all.

Measure the wait on the consumer side and report it in three ways:

• EXPLAIN ANALYZE prints "Cross-slice wait: N ms max (segK), M ms avg x P
  workers" under the node, for waits of 1 ms and above. Max against average
  shows whether one segment waited or all of them did.
• An errcontext callback attaches the elapsed wait to any error raised while
  blocked, which covers the statement_timeout cancellation.
• EState.es_cross_slice_wait holds the longest wait per query, and
  gp_stats_collector reports it as cross_slice_wait_ms.

Tests are added to src/test/regress/sql/shared_scan.sql for the planner and
the optimizer. The producer sleeps to force a wait above the threshold, and
a plpgsql wrapper checks only that the line is present, so no timing value
reaches the expected output. The metric is documented in
gpcontrib/gp_stats_collector/metric.md.

To reproduce:

create table sisw_foo (a int, b int) distributed by (a);
create table sisw_bar (c int, d int) distributed by (c);
insert into sisw_foo values (1, 2);
insert into sisw_bar select i, i from generate_series(1, 100) i;
analyze sisw_foo;
analyze sisw_bar;

set optimizer = off;
set gp_cte_sharing = on;

explain (analyze, costs off)
with cte as (select a, b, pg_sleep(0.5)::text as s from sisw_foo)
select x.a from cte x
union all
select y.a from cte y join sisw_bar on y.b = sisw_bar.c;

Only the segment holding the row sleeps, so its consumer blocks for half a
second while the others do not:

->  Shared Scan (share slice:id 2:0) (actual rows=1 loops=1)
      Cross-slice wait: 502.311 ms max (seg1), 0.240 ms avg x 2 workers.

Without the patch the node reports only its actual time, with the half
second charged to the scan itself.