check hook for anon in shared_preload_libraries for dynamic masking

Even though the documentation explicitly states multiple times that anon must be in shared_preload_libraries for dynamic masking to work, this requirement can slip through the cracks and be overlooked by users.

This PR adds a check_hook on the anon.transparent_dynamic_masking GUC that enables dynamic masking, which fails if anon is not in shared_preload_libraries.