Lazily retrieve EngineFactory based on IndexSettings

[dakrone]

This commit changes the way that EngineFactorys are returned, instead of the
EngineFactory being set at time of shard creation, this changes the pluggability
so that an existing IndexShard can return a different engine factory depending
on dynamic index settings.

Relates to #31141

[elasticmachine]

Pinging @elastic/es-distributed

[bleskes]

I would really like to try to avoid having an EngineFactoryFactory (effectively). Maybe we can change EngineFactory.newReadWriteEngine to get an index settings too?

[dakrone]

Maybe we can change EngineFactory.newReadWriteEngine to get an index settings too?

Sure, I think that might be an option, I'll make those changes. However, do we want to rename the method at the same time to just be newEngine(EngineConfig config)? It seems strange to me to be providing engines that may not be read/write from a method named "newReadWriteEngine"

[dakrone]

Actually, it looks like this is already possible with the EngineConfig containing the settings. I'll look into doing what I need without this change.

[dakrone]

Okay, I think we can get away without this. I opened #31745 so I'm closing this for now.