[discuss] dynamic settings

### Description
I am currently working on a feature which will be configurable through the settings and will eventually make sense to be able to dynamically change while logstash is running, given a UI and/or API. Same idea with runtime parameters like batch size or number or workers - il would be really useful to be able to change some of the runtime parameters dynamically. These are my thoughts after discussing this idea with @jsvd. #

### Proposal
- Most of LS runtime parameters are now managed with the `Settings` class. I propose we enhance/refactor this class to support multi-threading, where both settings reader threads (ex.: pipeline) and writer threads (ex.: UI or API) could concurrently read/write in the global settings.

- A callback mechanism could be introduced to notify of changed settings?

- Support for multiple settings instances, for example with multiple pipelines, would be required so that dynamic settings PER running pipeline would be supported.

- I think this Settings API should somehow also be exposed to **some** plugins so that plugins could change the LS runtime behaviour. This is obviously tricky and need to be properly assessed for security concerns.

I like this idea of completely decoupling the settings readers and writers and making it completely generic so that any upcoming UI/API would not need to have any specific internal class knowledge (pipeline, agent, logging, ...) and only manipulate this settings class.