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.