Support date math in index names

[smowky]

Is your feature request related to a problem? Please describe.
I’m using elasticsearch ILM to rotate my indexes and now alerts are searching through all indexes metricbeat-* which is quite expensive operation.
It would be nice to have a possibility to search only for last 2 day indexes instead

Describe the solution you'd like
Would be nice define search index as:
<logstash-{now/d-2d}>,<logstash-{now/d-1d}>,<logstash-{now/d}>
or
%3Clogstash-%7Bnow%2Fd-2d%7D%3E%2C%3Clogstash-%7Bnow%2Fd-1d%7D%3E%2C%3Clogstash-%7Bnow%2Fd%7D%3E

Describe alternatives you've considered
Alternative solutions is using aliases , which is be default not as easy since elasticsearch ILM actions does not have a possibility do add/remove alias

[qreshi]

Closing in favor of opensearch-project/alerting#74