pgsql: Add context type field to pg_backend_memory_contexts

Add context type field to pg_backend_memory_contexts

Since we now (as of v17) have 4 MemoryContext types, the type of context
seems like useful information to include in the pg_backend_memory_contexts
view.  Here we add that.

Reviewed-by: David Christensen, Michael Paquier
Discussion: https://postgr.es/m/CAApHDvrXX1OR09Zjb5TnB0AwCKze9exZN%3D9Nxxg1ZCVV8W-3BA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/12227a1d5f8ecad296c4204cc924d33c6a6bcd34

Modified Files
--------------
doc/src/sgml/system-views.sgml         |  9 +++++++++
src/backend/utils/adt/mcxtfuncs.c      | 35 +++++++++++++++++++++++++++-------
src/include/catalog/catversion.h       |  2 +-
src/include/catalog/pg_proc.dat        |  6 +++---
src/test/regress/expected/rules.out    |  3 ++-
src/test/regress/expected/sysviews.out | 16 ++++++++--------
src/test/regress/sql/sysviews.sql      |  4 ++--
7 files changed, 53 insertions(+), 22 deletions(-)