Support key_as_string inside aggregation path

[costin]

Multi-bucket aggregations that support formatting, like DateHistogram provide their content in a formatted manner (typically through key_as_string). While inside aggregation path one can refer to the key, currently there's no support for getting the formatted key despite being available.

[jpountz]

For the record, I'd like to remove key_as_string and fix key to always return formatted keys. (#18518)

[costin]

Sounds good to me.

[clintongormley]

Closing in favour of #18518