Automatically purge old keyrings from /etc/apt/keyrings

## Use Case
When creating keyrings through apt::source' `key` parameter there doesn't seem to be a way to automatically cleanup built-in to the apt module, so keyrings are left behind when removing sources.

## Describe the Solution You Would Like
Managing the directory and having a purge option as done for apt.conf.d and friends.

## Describe Alternatives You've Considered
none
