Add `config.active_record.permanent_connection_checkout` setting

Created by: casperisfine

Ref: https://github.com/rails/rails/pull/50793

Controls whether ActiveRecord::Base.connection raises an error, emits a deprecation warning, or neither.

ActiveRecord::Base.connection checkouts a database connection from the pool and keep it leased until the end of the request or job. This behavior can be undesirable in environments that use many more threads or fibers than there is available connections.

This configuration can be used to track down and eliminate code that calls ActiveRecord::Base.connection and migrate it to use ActiveRecord::Base.with_connection instead.

The default behavior remains unchanged, and there is currently no plans to change the default.

One thing that I think may be missing is a way to wrap legacy code with something akin to .with_connection, but that wouldn't cause .connection to persist the checkout. Perhaps that should be a .with_connection keyword argument?

@matthewd any opinion on the above?