pgsql: Support for unnest(multirange)

Support for unnest(multirange)

It has been spotted that multiranges lack of ability to decompose them into
individual ranges.  Subscription and proper expanded object representation
require substantial work, and it's too late for v14.  This commit
provides the implementation of unnest(multirange), which is quite trivial.
unnest(multirange) is defined as a polymorphic procedure.

Catversion is bumped.

Reported-by: Jonathan S. Katz
Discussion: https://postgr.es/m/flat/60258efe-bd7e-4886-82e1-196e0cac5433%40postgresql.org
Author: Alexander Korotkov
Reviewed-by: Justin Pryzby, Jonathan S. Katz, Zhihong Yu, Tom Lane
Reviewed-by: Alvaro Herrera

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/244ad5415557812a6ac4dc5d6e2ae908361d82c3

Modified Files
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doc/src/sgml/func.sgml                        | 23 +++++++++
src/backend/utils/adt/multirangetypes.c       | 73 +++++++++++++++++++++++++++
src/include/catalog/catversion.h              |  2 +-
src/include/catalog/pg_proc.dat               |  4 ++
src/test/regress/expected/multirangetypes.out | 24 +++++++++
src/test/regress/sql/multirangetypes.sql      |  7 +++
6 files changed, 132 insertions(+), 1 deletion(-)