pgsql: psql: Sprinkle more CppAsString2() in describe.c

psql: Sprinkle more CppAsString2() in describe.c

Like 91f5a4a000ea for pg_amcheck, this makes the code more
self-documented as there is less need to look in the headers what a
hardcoded value means. This touches queries related to procedures, AMs,
functions, databases, relations, constraints, collations, types and
extended stats, pulling into psql their *_d.h headers. The queries are
written the same way as originally.

There are still a couple of hardcoded values. These cannot be included
yet as they are not exposed in headers that are safe to use in frontend
code.

Note that describe.c was including pg_am.h that should be used only in
backend code. This is updated to use pg_am_d.h.

Reviewed-by: Daniel Gustafsson, Corey Huinker
Discussion: https://postgr.es/m/Zxb2hpca-pZc6zKe@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/18954ce7f69b0a4725b1cf1845ed5fd3874e2a92

Modified Files
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src/bin/psql/describe.c | 155 ++++++++++++++++++++++++++++++------------------
1 file changed, 98 insertions(+), 57 deletions(-)