Allow clients to pass in the `last_update_range` parameter in `search_datasets` call

[shahfazal]

The search_datasets tool does not expose the last_update_range parameter. Having the tool accept this parameter would allow clients to filter data being searched based on their recency and relevancy needs.

The parameter accepts last_30_days, last_12_months and last_3_years as values, as listed in the v2 swagger docs.

The search_datasets tool already invokes the v2 API, so this change would just add the new parameter with a default of None so as not to break existing clients or integrations.

[shahfazal]

This follows the discussion in #20 where @estellebertrand suggested last_update_range as a more precise alternative to sort-based filtering.