In #82860 we had a case when a plugin wanted to perform a custom logic on value change.

User can adjust UiSettings in different ways:

• in UI
• calling UiSettings REST API
• calling SO REST API
• changing kibana.yml

We want to make sure that we cover all of the causes.

There are several different options:

• move UiSettings change to the plugin context. Therefore, plugins could perform any custom logic and call UiSettings API to write/read UiSettings
• still rely on AdavancedSettings UI, but provide a way for plugins to hook in platform API. For example, provide a stream of write operations to a plugin-owner, which can be used to implement any plugin-specific logic on top of it.