[FEA] Support FIRST_VALUE and LAST_VALUE in grouped_rolling_window

**Is your feature request related to a problem? Please describe.**
Like ROW_NUMBER, which supported in PR 4881, FIRST_VALUE is widely used window function in sql. So, we want to provide it for BlazingSQL. FIRST_VALUE and LAST_VALUE are fairly trivial where there are no partitions to worry about, but if you have multiple groups or partitions, for the case of `grouped_rolling_window`, then its not so trivial, and we would want native support for that aggregator in cudf.

**Describe the solution you'd like**
Implementation of the aggregation for FIRST_VALUE and LAST_VALUE, especially for `grouped_rolling_window`