Introduce downsampling configuration for data stream lifecycle

[gmarouli]

This change adds support for downsampling lifecycle config like the following:

{
  "lifecycle": {
    "data_retention": "90d",
    "downsampling": [
      {
        "after": "1d",
        "fixed_interval": "2h"
      },
      { "after": "15d", "fixed_interval": "1d" },
      { "after": "30d", "fixed_interval": "1w" }
    ]
  }
}

We will also support saying that explicitly do not want any downsampling (for templates):

{
  "lifecycle": {
    "data_retention": "90d",
    "downsampling": null
  }
}

And that we do not have a preference for downsampling:

{
  "lifecycle": {
    "data_retention": "90d"
  }
}

Disclaimer
This proposal is an alternative implementation to #96848.

We propose to not add new functionality to AbstractObjectParser.java because our understanding is that the nullification needed for template composition is not something we want to have in general. We chose to expose the parsing of an array value, so this special array parsing can be contained within the DataLifecycle code.

Part of: #93596

[gmarouli]

@elasticmachine update branch

[andreidan]

This approach LGTM, thanks Mary
// cc @masseyke

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[elasticsearchmachine]

Hi @gmarouli, I've created a changelog YAML for you.

[andreidan]

Thanks for working on this Mary, this looks great

Left a couple of comments

[andreidan]

Could we use the existing DownsampleConfig instead of just a TimeValue for the fixedInterval?

We'll later be able to use it in the DownsampleAction#Request and implement the intervals validations similar to how we do it in https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/rollup/src/main/java/org/elasticsearch/xpack/downsample/TransportDownsampleAction.java#L527C43-L527C43

[gmarouli]

Definitely, I was so focused on how to parse it I completely forgot to check for existing code. Thanks @andreidan

[gmarouli]

I had to make some adjustments to the DownsamplingConfig to use it in DataLifecycle but I believe that they are not too invasive.

[andreidan]

Shall we also introduce the validation that all fixed_intervals are required to be multiples of each other?

(This could be a follow-up as well)

[gmarouli]

I will give it a try.

[gmarouli]

I extrcted this validation so now it's shared.

[andreidan]

LGTM, this is great, thanks Mary

[gmarouli]

Thank you for the quick review and the sharp comments! 🚀