add support for index templates other than logs and metrics as types when using data streams

[maxgruber19]

In most real world scenarios a user is not in control over the default index templates "logs" and "metrics". There are many usecases where the user wants to define a custom template. In my case I want to use the data retention feature to delete messages stored in elasticsearch after a certain amount of time.

Currently this connector is able to use the predefined index templates "logs" and "metrics" when data streams are enabled. User created templates are not supported yet.

I'd like to contribute some lightweight lines to make the connector support custom templates as well.

I guess issue #700 tried to do the same but I didn't find a related pullrequest or reactions on that thread.