Add close_timeout config to filebeat

Hi,

As reported https://bugs.launchpad.net/filebeat-charm/+bug/1875662, we're seeing few instances where filebeat is holding onto older log files. @jetpackdanger pointed out https://www.elastic.co/guide/en/beats/filebeat/current/faq-deleted-files-are-not-freed.html

Any chance we can add a option to tune Filebeat's close_timeout (and possibly set a sensible default such as an hour or two)?

Start by reading the filebeat charm's existing configuration options and the Filebeat FAQ linked in the issue. Trace how charm options become Filebeat settings, then make close_timeout configurable with the agreed default and verify that the generated configuration uses it.

Written by the indexing model from the issue text.