Include time series temporal ranges to get data stream api for data streams in time series index mode

[martijnvg]

Change the get data stream api to include the following properties of a data stream:

• Add index_mode from DataStream to the response.
• For each backing index of a data stream with time series index mode, include the temporal range the backing index represents (based on the index.time_series.start_time and index.time_series.end_time index settings.

[elasticmachine]

Pinging @elastic/es-data-management (Team:Data Management)