Not allowing index starting with . for replication

[naveenpajjuri]

Description

Blocking indices that start with . for replication.

Sample response

{
   "error": {
       "root_cause": [
           {
               "type": "invalid_index_name_exception",
               "reason": "Invalid index name [.check6], Cannot start replication for an index starting with '.'",
               "index_uuid": "_na_",
               "index": ".check6"
           }
       ],
       "type": "invalid_index_name_exception",
       "reason": "Invalid index name [.check6], Cannot start replication for an index starting with '.'",
       "index_uuid": "_na_",
       "index": ".check6"
   },
   "status": 400
}

Issues Resolved

[List any issues this PR will resolve]

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[dblock]

Curious, why is this a thing? Is this convention of being a system index documented and do we explain why these shouldn't be replicated?