Introduce gpu kwarg to Context.__init__, deprecate for create_table

[charlesbluca]

Having a GPU flag set at the context level should allow us to more easily make assumptions when doing things such as:

• creating tables on the fly (i.e., AnalyzeTable or EmptyRelation plugins)
• setting SQL to Python type mappings (xref Implement initial decimal support #1073)
• tune queries mid-execution to optimize for performance

Assume the majority of the work here will be test refactoring to get things passing, but hopeful that this will make it a little easier to generalize CPU/GPU tests moving forward.

[codecov-commenter]

Codecov Report

Merging #1117 (a3019a2) into main (c8a3234) will decrease coverage by 0.24%.
The diff coverage is 72.41%.

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@@            Coverage Diff             @@
##             main    #1117      +/-   ##
==========================================
- Coverage   81.76%   81.52%   -0.24%     
==========================================
  Files          78       78              
  Lines        4397     4443      +46     
  Branches      792      801       +9     
==========================================
+ Hits         3595     3622      +27     
- Misses        631      637       +6     
- Partials      171      184      +13     

Impacted Files	Coverage Δ
dask_sql/physical/rel/logical/aggregate.py	88.35% <0.00%> (-2.90%)	⬇️
dask_sql/utils.py	96.49% <50.00%> (-2.59%)	⬇️
dask_sql/context.py	91.91% <63.15%> (-1.87%)	⬇️
dask_sql/physical/rel/custom/analyze_table.py	86.95% <100.00%> (-2.33%)	⬇️
dask_sql/physical/rel/custom/describe_model.py	87.50% <100.00%> (+0.54%)	⬆️
dask_sql/physical/rel/custom/show_columns.py	86.95% <100.00%> (+0.59%)	⬆️
dask_sql/physical/rel/custom/show_models.py	82.35% <100.00%> (+1.10%)	⬆️
dask_sql/physical/rel/custom/show_schemas.py	88.00% <100.00%> (+0.50%)	⬆️
dask_sql/physical/rel/custom/show_tables.py	86.95% <100.00%> (+0.59%)	⬆️
dask_sql/physical/rel/logical/empty.py	82.35% <100.00%> (+1.10%)	⬆️

... and 5 files with indirect coverage changes

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