This commit adds telemetry for our data tier formalization. This telemetry helps determine the
topology of the cluster with regard to the content, hot, warm, & cold tiers/roles.

An example of the telemetry looks like:

```
GET /_xpack/usage?human
{
  ...
  "data_tiers" : {
    "available" : true,
    "enabled" : true,
    "data_warm" : {
      ...
    },
    "data_cold" : {
      ...
    },
    "data_content" : {
      "node_count" : 1,
      "index_count" : 6,
      "total_shard_count" : 6,
      "primary_shard_count" : 6,
      "doc_count" : 71,
      "total_size" : "59.6kb",
      "total_size_bytes" : 61110,
      "primary_size" : "59.6kb",
      "primary_size_bytes" : 61110,
      "primary_shard_size_avg" : "9.9kb",
      "primary_shard_size_avg_bytes" : 10185,
      "primary_shard_size_median" : "8kb",
      "primary_shard_size_median_bytes" : 8254,
      "primary_shard_size_mad" : "7.2kb",
      "primary_shard_size_mad_bytes" : 7391
    },
    "data_hot" : {
       ...
    }
  }
}
```

The fields are as follows:

- node_count :: number of nodes with this tier/role
- index_count :: number of indices on this tier
- total_shard_count :: total number of shards for all nodes in this tier
- primary_shard_count :: number of primary shards for all nodes in this tier
- doc_count :: number of documents for all nodes in this tier
- total_size_bytes :: total number of bytes for all shards for all nodes in this tier
- primary_size_bytes :: number of bytes for all primary shards on all nodes in this tier
- primary_shard_size_avg_bytes :: average shard size for primary shard in this tier
- primary_shard_size_median_bytes :: median shard size for primary shard in this tier
- primary_shard_size_mad_bytes :: [median absolute deviation](https://en.wikipedia.org/wiki/Median_absolute_deviation) of shard size for primary shard in this tier

Relates to elastic#60848