[#7070] ysql: Import 'Fix ancient memory leak in contrib/auto_explain.'

Summary:
Upstream commit was 5c0f7cc5442108e113d4fb88c952329b467e2c6a

Commit message was:
```
    The ExecutorEnd hook is invoked in a context that could be quite
    long-lived, not the executor's own per-query context as I think
    we were sort of assuming.  Thus, any cruft generated while producing
    the EXPLAIN output could accumulate over multiple queries.  This can
    result in spectacular leakage if log_nested_statements is on, and
    even without that I'm surprised nobody complained before.

    To fix, just switch into the executor's context so that anything we
    allocate will be released when standard_ExecutorEnd frees the executor
    state.  We might as well nuke the code's retail pfree of the explain
    output string, too; that's laughably inadequate to the need.

    Japin Li, per report from Jeff Janes.  This bug is old, so
    back-patch to all supported branches.

    Discussion: https://postgr.es/m/CAMkU=1wCVtbeRn0s9gt12KwQ7PLXovbpM8eg25SYocKW3BT4hg@mail.gmail.com
```

Test Plan: Build Yugabyte DB and run test suite via Jenkins

Reviewers: jason

Reviewed By: jason

Subscribers: yql

Differential Revision: https://phabricator.dev.yugabyte.com/D10529

src/postgres/contrib/auto_explain/auto_explain.c

@@ -309,8 +309,15 @@ explain_ExecutorEnd(QueryDesc *queryDesc)
 {
 	if (queryDesc->totaltime && auto_explain_enabled() && current_query_sampled)
 	{
+		MemoryContext oldcxt;
 		double		msec;
 
+		/*
+		 * Make sure we operate in the per-query context, so any cruft will be
+		 * discarded later during ExecutorEnd.
+		 */
+		oldcxt = MemoryContextSwitchTo(queryDesc->estate->es_query_cxt);
+
 		/*
 		 * Make sure stats accumulation is done.  (Note: it's okay if several
 		 * levels of hook all do this.)
@@ -360,9 +367,9 @@ explain_ExecutorEnd(QueryDesc *queryDesc)
 					(errmsg("duration: %.3f ms  plan:\n%s",
 							msec, es->str->data),
 					 errhidestmt(true)));
-
-			pfree(es->str->data);
 		}
+
+		MemoryContextSwitchTo(oldcxt);
 	}
 
 	if (prev_ExecutorEnd)