[SIEM][Detection Engine] Create signals mapping/index template on startup

[FrankHassanabad]

See here:
https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-templates.html

We need to initialize a template on startup of a Kibana instance. There can be more than 1 instance of Kibana starting close to each other and we will need to query for a check against the existence of it before adding it.

You can see the current template examples like so:

GET /_template

More than likely we should start out with an index named .siem-signals- since that would be similar to the ".ml-anomalies-" template.

For the ILM policies see here:
https://www.elastic.co/guide/en/elasticsearch/reference/current/getting-started-index-lifecycle-management.html

Choose to start:

50GB or 30 days rotation, no deletion

[elasticmachine]

Pinging @elastic/siem

[FrankHassanabad]

This is completed and there might be tweaks but this should be closed now.