[FEATURE] Support fetching bulk alerts for multiple monitors in get alerts API

[eirsep]

Get alerts API in Alerting plugin supports fetching alerts for a given monitor id or fetches all alerts present in an index.

Some use cases would require fetching alerts for a given list of monitors rather than one.

In the current implementation this implies n calls to alerting plugin for n monitors which translates to n search requests.

We need to enhance the API to accept list of monitors and that way can boil it down to one search request.

[eirsep]

The only caveat to keep in mind is currently we can only fetch from one index.

[eirsep]

@getsaurabh02 @lezzago @sbcd90