[ILM] Settings Template

[pickypg]

For years, it has been common to set specific settings during index curation (and creation, of course) for different stages the index lifecycle.

Depending on your use case, the settings change based on the stage and scale. In Curator, we had to explicitly call out settings and ILM is similar, but the ability to set certain settings is more restricted (see #44070 as an example).

I propose a Settings Template, which can be consumed by ILM and Index/Component Templates. Its entire purpose is two-fold:

1. Explicitly separate creation-only settings and resettable settings. This avoids the pitfalls of thinking you're going to be able to change something later that can only be set during index creation.
2. Enable ILM to set arbitrary settings at any stage of the ILM shifts other than deletion

This should enable even more transposable template usage, as well as stop requiring ILM to explicitly support settings.

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)