SQL Property Graph Queries (SQL/PGQ)

SQL Property Graph Queries (SQL/PGQ)

Implementation of SQL property graph queries, according to SQL/PGQ
standard (ISO/IEC 9075-16:2023).

This adds:

- GRAPH_TABLE table function for graph pattern matching
- DDL commands CREATE/ALTER/DROP PROPERTY GRAPH
- several new system catalogs and information schema views
- psql \dG command
- pg_get_propgraphdef() function for pg_dump and psql

A property graph is a relation with a new relkind RELKIND_PROPGRAPH.
It acts like a view in many ways.  It is rewritten to a standard
relational query in the rewriter.  Access privileges act similar to a
security invoker view.  (The security definer variant is not currently
implemented.)

Starting documentation can be found in doc/src/sgml/ddl.sgml and
doc/src/sgml/queries.sgml.

Author: Peter Eisentraut <peter@eisentraut.org>
Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Ajay Pal <ajay.pal.k@gmail.com>
Reviewed-by: Henson Choi <assam258@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/a855795d-e697-4fa5-8698-d20122126567@eisentraut.org