[FEATURE] Add leader_alias to autofollower status

[tmanninger]

Is your feature request related to a problem? The output auf the "API" '_plugins/_replication/autofollow_stats' does not return the name of the leader_alias

What solution would you like? See "leader_alias"

curl -XGET -u 'admin:<custom-admin-password>' -k 'https://localhost:9200/_plugins/_replication/autofollow_stats'

{
   "num_success_start_replication": 1,
   "num_failed_start_replication": 0,
   "num_failed_leader_calls": 0,
   "failed_indices":[
      
   ],
   "autofollow_stats":[
      {
         "name":"my-replication-rule",
         "pattern":"movies*",
         "leader_alias": "my-connection-alias"   <--- add this line to the API output
         "num_success_start_replication": 1,
         "num_failed_start_replication": 0,
         "num_failed_leader_calls": 0,
         "failed_indices":[
            
         ]
      }
   ]
}