Special case Elasticsearch 7.8+ Composable index templates

[codefromthecrypt]

Feature: If we prefer composable templates or otherwise prioritize our index templates, we can avoid support problems like https://github.com/openzipkin/zipkin-support/issues/23 where the template doesn't kick in due to something else matching on '*'

Rational Composeable templates are both introduced and preferred in ES 7.8+, which refers to _template as legacy ones.

This would introduce an issue of composeable template priority, either it would have to determine automatically what value should be set or be parameterizable through env variable.

Example Scenario Test would have a pre-existing composable template and ours still applies

Thanks @benqs for the hint. cc @openzipkin/elasticsearch

[xeraa]

BTW this is part of a bigger change around time-based indices that will lead to data streams; though that is still work in progress. So I would wait a little longer here and see if that might make sense for Zipkin as well.