Support Postgresql 15 features

https://www.postgresql.org/about/news/postgresql-15-released-2526/

PostgreSQL 15 includes the SQL standard MERGE command. MERGE lets you write conditional SQL statements that can include INSERT, UPDATE, and DELETE actions within a single statement.

This latest release adds new functions for using regular expressions to inspect strings: regexp_count(), regexp_instr(), regexp_like(), and regexp_substr(). PostgreSQL 15 also extends the range_agg function to aggregate multirange data types, which were introduced in the previous release.

PostgreSQL 15 lets users create views that query data using the permissions of the caller, not the view creator. This option, called security_invoker, adds an additional layer of protection to ensure that view callers have the correct permissions for working with the underlying data.

Merge in paticular looks awesome. It's a SQL standard, so we may be able to support it in all database backends, too. https://www.postgresql.org/docs/15/sql-merge.html

Start with the PostgreSQL 15 release notes and the linked MERGE documentation, then locate the persistent database-backend entry points that define supported SQL features. Determine which of MERGE, regular-expression functions, multirange aggregation, and security_invoker views are in scope. Done means the selected PostgreSQL 15 features are supported with appropriate backend coverage.