Enhanced age filter to perform limit calcs internally using dynamically fetched limit setting

[richard-mauri]

This enhancement to the logstash age filter plugin optionally invokes a remote "limit service" to acquire a limit setting. The limit service can be polled at a scheduled interval.
The age plugin then performs the caculation if the event is older than that limit and sets a field if it is or not.
The

This was motivated by the need to be able to remove hard-coded limit setttings and associated calculation from the pipeline filter config file, and instead query and dynamically the limit settings without changing a config file and restarting or signalling logstash.

It also demonstrates how to use the elasticsearch cluster settings metadata as the limit service.

[richard-mauri]

I know the ascidoc needs attention; I'm not sure how to update it from the comments in the source file.
I hate to copy/paste.

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
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Please, read and sign the above mentioned agreement if you want to contribute to this project

[richard-mauri]

I filed a contributor agreement on another pull request.
It goes nowhere; in other words the response to the contributor form says "elastic will get back to you once it is signed by everybody". Well, it must get stalled as I never get a confirmation and there is no link to track the state of the confirmation....

[richard-mauri]

I had to receate the repo: It is here: https://github.com/richard-mauri/logstash-filter-age

[richard-mauri]

closing and opening a new pr