Add support for gpu kwarg in Context.sql and explain

[charlesbluca]

While working on #365, I noticed that Context.sql and explain both allow dataframes to be input directly, prompting the corresponding create_table calls before executing.

This PR adds the gpu kwarg to these methods, to be passed to the underlying create_table calls.

[ayushdg]

Changes look good to me and should be good to merge once CI passes.

[codecov-commenter]

Codecov Report

Merging #368 (70e89e4) into main (6040f00) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #368   +/-   ##
=======================================
  Coverage   95.57%   95.57%           
=======================================
  Files          67       67           
  Lines        2935     2935           
  Branches      547      547           
=======================================
  Hits         2805     2805           
  Misses         79       79           
  Partials       51       51           

Impacted Files	Coverage Δ
dask_sql/context.py	100.00% <100.00%> (ø)

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