[DF] Add single_value window function

[ayushdg]

Is your feature request related to a problem? Please describe. dask-sql previously supported a single_value window function that was functionally identical to first_value. Which would fail with the datafusion branch

Describe the solution you'd like Since this function is not standard postresql syntax we should look into registering it as a custom function if possible.

Describe alternatives you've considered N/A

Additional context N/A