This commit adds a new endpoint for Rollup V2, a new way to rollup indices.

Instead of relying on a cron-job, this action will rollup a whole index on the spot.

When an index is rolled up using a Rollup Config, it does the following

1. check that original index is read-only
2. runs an aggregation and indexes results into a temporary hidden rollup index
3. "resizes" the temporary index into the final rollup-index (in-place segment pointer juggling)
4. adds RollupMetadata about the rollup-group (keyed by original index name) and adds
    custom index-metadata with data about what the rollup index's original index is so that its
    group information in RollupMetadata can be looked up

example usage:

```
POST /_rollup_vtwo/index
{
    "rollup_index": "index_rolled",
    "groups": {
        "date_histogram": {
            "field": "date",
            "calendar_interval": "1M"
        },
        "terms": {
            "fields": ["unit"]
        }
    },
    "metrics": [
        {
            "field": "temperature",
            "metrics": ["sum"]
        }
    ]
}
```

relates elastic#42720.

This commit adds a new endpoint for Rollup V2, a new way to rollup indices.

Instead of relying on a cron-job, this action will rollup a whole index on the spot.

When an index is rolled up using a Rollup Config, it does the following

1. check that original index is read-only
2. runs an aggregation and indexes results into a temporary hidden rollup index
3. "resizes" the temporary index into the final rollup-index (in-place segment pointer juggling)
4. adds RollupMetadata about the rollup-group (keyed by original index name) and adds
    custom index-metadata with data about what the rollup index's original index is so that its
    group information in RollupMetadata can be looked up

example usage:

```
POST /_rollup_vtwo/index
{
    "rollup_index": "index_rolled",
    "groups": {
        "date_histogram": {
            "field": "date",
            "calendar_interval": "1M"
        },
        "terms": {
            "fields": ["unit"]
        }
    },
    "metrics": [
        {
            "field": "temperature",
            "metrics": ["sum"]
        }
    ]
}
```

relates #42720.

This PR implements downsampling operation on time series indices.

The PR creates a _rollup endpoint that allows users to downsample an index and can be
accessed by the following call:

POST /<source_index>/_rollup/<rollup_index>
{
    "fixed_interval": "1d"
}

Requirements

An index can be downsampled if all of the following requirements are met:

    Must be a time series index (have the index.mode: time_series index setting)
    Must not be writeable (have the index.blocks.write: true index setting)
    Must have dimension fields marked with mapping parameter time_series_dimension: true
    Must have metric fields marked with mapping parameter time_series_metric

Relates to #74660

Fixes #65769
Fixes #69799
Finally, this PR is based on the code written for #64900

This PR implements downsampling operation on time series indices.

The PR creates a _rollup endpoint that allows users to downsample an index and can be
accessed by the following call:

POST /<source_index>/_rollup/<rollup_index>
{
    "fixed_interval": "1d"
}

Requirements

An index can be downsampled if all of the following requirements are met:

    Must be a time series index (have the index.mode: time_series index setting)
    Must not be writeable (have the index.blocks.write: true index setting)
    Must have dimension fields marked with mapping parameter time_series_dimension: true
    Must have metric fields marked with mapping parameter time_series_metric

Relates to elastic#74660

Fixes elastic#65769
Fixes elastic#69799
Finally, this PR is based on the code written for elastic#64900