Add new is_write_index flag to Alias MetaData

[talevy]

in response to #30061.

This PR rebuilds aliasAndIndexLookup every time it wants to use it for alias validation. Depends on outcome of #29575 for using a pre-built data-structure.

This PR introduces a concept in aliases called is_write_index. When true on a specific
alias definition associated with an index, index requests using the associated alias name will
resolve to this index for indexing.

Write now, if one attempts to index a document using an alias that points to multiple indices, there is no way to resolve which one, so an exception is thrown. With this flag, you can have an alias that points to a specific index in a set of indices as the one to write to.

It is possible to set an index as an alias' write-index during index creation:

PUT foo1
{
  "aliases": {
    "bar": { "is_write_index": true }
  }
}

Now, index requests like PUT bar/_doc/_id will route to index foo1. Another index, foo2, is
free to also have an alias named "bar" pointing to it. This alias needs to be configured with is_write_index set to false. is_write_index is defaulted to false if not specified, or it can be explicitly set to false.

PUT foo2
{
  "aliases": {
    "bar": {}
  }
}

Now, searches against bar will point to both indices, but index requests will be routed to foo1.

These settings can be updated in the _aliases API like so:

POST _aliases
{
  "actions": [
    {
      "add": {
        "index": "foo1",
        "alias": "bar",
        "is_write_index": false
      }
    }
  ]
}

How does this relate to Rollover? Rollover depended on rollover-aliases that only point to
one index since multiple would break indexing. Now that the same alias can point to multiple indices, the only requirement we have is that there is only one write index.

Here is an example scenario that didn't work before, but works with these changes:

PUT _template/my_template
{
  "index_patterns": ["foo-*"],
  "settings": {
    "number_of_shards": 1,
    "number_of_replicas": 1
  },
  "aliases": {
    "logs": {}
  },
  "mappings": {
    "_doc": {
      "_source": {
        "enabled": false
      }
    }
  }
}

PUT foo-000001
{
  "aliases": {
    "logs": { "is_write_index": true }
  }
}

PUT logs/_doc/_id
{
  "hello": "world"
}

POST /logs/_rollover/
{
  "conditions": {
    "max_docs":  1
  }
}

GET foo-000001 # now this is `is_write_index: false`
GET foo-000002 # now this is `is_write_index: true`

[elasticmachine]

Pinging @elastic/es-distributed

[elasticmachine]

Pinging @elastic/es-core-infra

[talevy]

Closing in favor of new PR

[talevy]

I've reopened a more complete version of this feature here: #30703