Hooking into ExplainOneQuery() complicated by missing standard_ExplainOneQuery

Hi hackers,

I wanted to hook into the EXPLAIN output for queries and add some extra
information, but since there is no standard_ExplainOneQuery() I had to copy
the code and create my own version.

Since the pattern with other hooks for a function WhateverFunction() seems
to be that there is a standard_WhateverFunction() for each
WhateverFunction_hook, I created a patch to follow this pattern for your
consideration.

I was also considering adding a callback so that you can annotate any node
with explanatory information that is not a custom scan node. This could be
used to propagate and summarize information from custom scan nodes, but I
had no immediate use for that so did not add it here. I would still be
interested in hearing if you think this is something that would be useful
to the community.

Best wishes,
Mats Kindahl, Timescale