[WIP] Add DownSampleAction to downsampling time_series index

[weizijun]

I add a new action named DownSampleAction. It's used to downsample time_series index.
The API is a simplification of RollupAction:

POST /{index}/_downsample/{downsample_index}
{
  "fixed_interval": "10m"
}

All parameters are as follows:

{
  "fixed_interval": "xxx",
  "calendar_interval": "xxx",
  "time_zone": "xxx",
  "settings": {
    "index.number_of_shards": "2"
  }
}

Only one interval can be configure in calendar_interval and fixed_interval.

The DownSampleAction only supports time_series index.
It builds the RollupActionConfig from time_series index mapping, and calls the RollupAction.
the build logic is:

• build the date_histogram config from the down sample configure, and the field name is @timestamp
• find the time_series_dimension = true field, and add to terms group
• find the time_series_metric field, and add to metrics group

The code is a POC, I didn't finish and test it. If the design is ok, I'll go ahead and finish it.

[csoulios]

Closing this PR because it overlaps with the work at PR #85708