[BUG] Alias filter updates don't get propagated to follower clusters

[krishna-ggk]

What is the bug?

Alias filter changes aren't getting replicated to follower cluster.

On inspecting code, the logic to replicate alias is missing code to replicate filter. This results in inconsistent alias on the follower end whenever an alias filter is updated.

                    for (alias in toAdd) {
                        log.info("Adding alias ${alias.alias} from $followerIndexName")
                        // Copying writeIndex from leader doesn't cause any issue as writes will be blocked anyways
                        request.addAliasAction(AliasActions.add().index(followerIndexName)
                                .alias(alias.alias)
                                .indexRouting(alias.indexRouting)
                                // Missing filter
                                .searchRouting(alias.searchRouting)
                                .writeIndex(alias.writeIndex())
                                .isHidden(alias.isHidden)
                        )
                    }

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Created a simple index with term filter on leader

curl -XPOST "$LEADER/_aliases" \
-H 'Content-Type: application/json' \
-d '{
  "actions": [
    {
      "add": {
        "index": "test_index",
        "alias": "test_alias",
        "filter": {
          "term": {
            "value": "8"
          }
        }
      }
    }
  ]
}'

2. Start replication.

curl -XPUT "${FOLLOWER}/_plugins/_replication/test_index/_start?pretty" -H 'Content-Type: application/json' -d '{"leader_alias":"my-leader-1", "leader_index": "test_index", "use_roles": { "leader_cluster_role": "all_access", "follower_cluster_role": "all_access" } }'

3. Check alias on follower

curl -XGET "$FOLLOWER/_alias/test_alias?pretty"

{
    "test_index": {
        "aliases": {
            "test_alias": {
                "filter": {
                    "term": {
                        "value": "8"
                    }
                }
            }
        }
    }
}

4. Now change the alias filter term query

curl -XPOST "$LEADER/_aliases" \
-H 'Content-Type: application/json' \
-d '{
  "actions": [
    {
      "add": {
        "index": "test_index",
        "alias": "test_alias",
        "filter": {
          "term": {
            "value": "6"
          }
        }
      }
    }
  ]
}'

5. Check alias on follower

curl -XGET "$FOLLOWER/_alias/test_alias?pretty"

## Got 404 a few times and later got following

{
    "test_index": {
        "aliases": {
            "test_alias": {}
        }
    }
}

What is the expected behavior?
The term filter changes should propagate to follower.