Pass return_type through to meta in apply

[brandon-b-miller]

UDFs that call apply currently trigger a dask warning since the return type is not packed into the meta that the final apply call uses. This PR passes the original return type passed by the user into apply to prevent the warning.

[codecov-commenter]

Codecov Report

Merging #275 (37d66fc) into main (b42db23) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #275   +/-   ##
=======================================
  Coverage   95.81%   95.82%           
=======================================
  Files          64       64           
  Lines        2799     2800    +1     
  Branches      424      424           
=======================================
+ Hits         2682     2683    +1     
  Misses         75       75           
  Partials       42       42           

Impacted Files	Coverage Δ
dask_sql/context.py	99.09% <100.00%> (ø)
dask_sql/datacontainer.py	94.11% <100.00%> (+0.07%)	⬆️

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[quasiben]

Thanks @brandon-b-miller