Implement retention of snapshots based on the document's timestamp date

[dakrone]

This is a spinoff of #43663,

With SLM retention we will allow retention for snapshots based on the snapshot's date, but it would be nice if we could also allow retention of snapshots based on the dates from the documents indices of the indices. This would allow a user to say something like "delete snapshots that have data with the @timestamp field older than 30 days".

[elasticmachine]

Pinging @elastic/es-core-features

[dakrone]

This has been open for quite a while, and we haven't made much progress on this due to focus in other areas. For now I'm going to close this as something we aren't planning on implementing. We can re-open it later if needed.