Migrate SKIPIF -> EXTENSIONS (#7138)

This is an automated migration of most SKIPIF extension_loaded checks.

Add GC support for PDO driver data

Add a get_gc method that can be implemented by drivers, which can
be used to add additional zvals to the GC buffer.

Implement GC support for P

Add GC support for PDO driver data

Add a get_gc method that can be implemented by drivers, which can
be used to add additional zvals to the GC buffer.

Implement GC support for PDO SQLite callbacks in particular.

Closes GH-6262.

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