[PATCH] Make ExplainOpenGroup()/ExplainCloseGroup() available for extensions.

Hello Hackers,

The attached patch moves declarations of
ExplainOpenGroup()/ExplainCloseGroup() from explain.c to explain.h.

This can be useful for extensions that need explain groups in their
custom-scan explain output.

For example, Citus uses groups in its custom explain outputs [1]. But it
achieves it by having a copy of ExplainOpenGroup()/ExplainCloseGroup() in
its source code, which is not the best way.

Please review.

[1] https://github.com/citusdata/citus/blob/master/src/backend/
distributed/planner/multi_explain.c

Thanks,
Hadi