pgsql: Add EXPLAIN (IO) instrumentation for SeqScan

Add EXPLAIN (IO) instrumentation for SeqScan

Adds support for EXPLAIN (IO) instrumentation for sequential scans. This
requires adding shared instrumentation, using the separate DSM approach
introduced by dd78e69cfc33.

Author: Tomas Vondra <tomas(at)vondra(dot)me>
Reviewed-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Lukas Fittl <lukas(at)fittl(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/flat/a177a6dd-240b-455a-8f25-aca0b1c08c6e%40vondra.me

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3b1117d6e2e47d86cdbd978b79434c630cb0ef52

Modified Files
--------------
src/backend/commands/explain.c | 25 +++++++
src/backend/executor/execParallel.c | 11 +++
src/backend/executor/nodeSeqscan.c | 133 ++++++++++++++++++++++++++++++---
src/include/executor/instrument_node.h | 19 +++++
src/include/executor/nodeSeqscan.h | 9 +++
src/include/nodes/execnodes.h | 1 +
src/test/regress/expected/explain.out | 18 ++++-
src/test/regress/sql/explain.sql | 4 +-
src/tools/pgindent/typedefs.list | 2 +
9 files changed, 209 insertions(+), 13 deletions(-)