Add index routing based on trace.id

Opening this up for discussion:

Should we start routing documents based on `trace.id`.
The thinking here is that we can always rely on a full trace being available in a single shard.

This potentially has the following benefits:

* Can use search time routing to get the data for trace waterfall
* We can reduce trace paths during the `reduce` phase of a scripted metric aggregation.
* Potentially faster joins with ESQL?
* Faster sequencing with EQL?

Unknowns:

* Datastreams require an [explicit opt in to enable routing](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-put-template.html#put-index-template-api-request-body), what are the consequences of opting in.