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.
