pgsql: Reintroduce support for sequences in pgstattuple and pageinspect

Reintroduce support for sequences in pgstattuple and pageinspect.

Commit 4b82664156 restricted a number of functions provided by
contrib modules to only relations that use the "heap" table access
method.  Sequences always use this table access method, but they do
not advertise as such in the pg_class system catalog, so the
aforementioned commit also (presumably unintentionally) removed
support for sequences from some of these functions.  This commit
reintroduces said support for sequences to these functions and adds
a couple of relevant tests.

Co-authored-by: Ayush Vatsa
Reviewed-by: Robert Haas, Michael Paquier, Matthias van de Meent
Discussion: https://postgr.es/m/CACX%2BKaP3i%2Bi9tdPLjF5JCHVv93xobEdcd_eB%2B638VDvZ3i%3DcQA%40mail.gmail.com
Backpatch-through: 12

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8a94af8a2d0f91ba9dbfb41a0c205e60afa509b2

Modified Files
--------------
contrib/pageinspect/expected/page.out        |  9 +++++++++
contrib/pageinspect/heapfuncs.c              |  6 +++++-
contrib/pageinspect/sql/page.sql             |  5 +++++
contrib/pgstattuple/expected/pgstattuple.out | 24 ++++++++++++++++++++++++
contrib/pgstattuple/pgstattuple.c            |  6 +++++-
contrib/pgstattuple/sql/pgstattuple.sql      | 12 ++++++++++++
6 files changed, 60 insertions(+), 2 deletions(-)