Add functionality to delete key providers

Providers can be deleted by the 

pg_tde_drop_key_provider([PG_TDE_GLOBAL, ] provider_name) 

Function.

For database specific providers, the function first checks if the provider is used or not, and the provider is only deleted if it’s not used.

For global providers, the function checks if the provider is used anywhere, WAL or any specific database, and returns an error if it is.

This somewhat goes against the principle that pg_tde shouldn’t interact with other databases than the one the user is connected to, but on the other hand, it only does this lookup in the internal pg_tde metadata, not in postgres catalogs, so it is a gray zone. Making this check makes more sense than potentially making some databases inaccessible.