Support using custom schema when creating extension

pg_cron always uses a schema named "cron" to house it's tables and functions. If you run `create extension pg_cron schema extension_pg_cron`, it still uses it's default cron schema, ignoring the specified desired schema. I think that pg_cron should work like other extensions, defaulting to the public schema and allowing users to override this default by specifying a schema on the create extension command.

Read the extension creation behavior and existing schema setup first; no file or test is named in the issue, so locate those entry points in the repository. Done means pg_cron uses public by default and honors a schema supplied to CREATE EXTENSION, with coverage for both cases.

Written by the indexing model from the issue text.