[Filebeat] Add persistent Netflow template cache

[fredbcode]

Hi all,

I found no way to use an netflow cache, cache_save_path (logstash syntax), in netflow
I don't know if this option is really missing or I just found no documentation about

With logstash I used a netflow.erb file https://github.com/elastic/logstash/blob/master/modules/netflow/configuration/logstash/netflow.conf.erb

With cache_ttl, cache_save_path and netflow_definitions added and all works well

related with #14617 ?

Filebeat version: 7.4.2

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)

[shipler]

@andrewkroh is there any expected update for the persistant template loading in the beats netflow module for the newer versions?

[jlind23]

Backlog grooming: Closing it for now until further activity.