[PATCH] Make ExplainBeginGroup()/ExplainEndGroup() public.

Hello,

The attached patch moves declarations of
ExplainBeginGroup()/ExplainEndGroup() 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
ExplainBeginGroup()/ExplainEndGroup() 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