Add S3 snapshot repository retention policy

[newgene]

Elasticsearch snapshot supports lifecycle management policy (SLM), we can set a policy like this:

"retention": {                    
    "expire_after": "30d",
    "min_count": 5,
    "max_count": 50
  }

Let's add it to hub config as an optional feature, likely in SNAPSHOT_CONFIG, and set it during the creation of a snapshot repository.

REF: https://www.elastic.co/guide/en/elasticsearch/reference/current/snapshots-take-snapshot.html#automate-snapshots-slm