Elasticsearch: Add backend support for multiple indexes

[matyax]

Initially reported here. When Elastic is in frontend mode it supports a comma-separated list of indexes, but when it's called as a backend data source it does not.

E.g. the data source settings test succeeds.

But then you get no logs in Explore.

As a side effect, this is affecting alerting.

[gabor]

hey all, so... i clicked [revert] on #67276 , which created #71284, because technically it introduces a bug. i recommend we merge that revert, and then we can discuss if we want to do something more or better. this way we can separate the:

• bugfix (this needs backporting)
• maybe an improvement

WDYT?

if you agree you can approve the revert PR 😁

[matyax]

I think this is done. Reopen otherwise.