Add template_location flag to Go Dataflow runner

[jrmccluskey]

Adds the --template_location flag and uses it to route the serialized job graph to an alternative GCS bucket instead of staging it and submitting the job to Dataflow.

Closes #21773

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Codecov Report

Merging #21774 (157ada1) into master (67533d1) will decrease coverage by 0.00%.
The diff coverage is 9.09%.

@@            Coverage Diff             @@
##           master   #21774      +/-   ##
==========================================
- Coverage   74.03%   74.02%   -0.01%     
==========================================
  Files         698      698              
  Lines       92192    92203      +11     
==========================================
+ Hits        68255    68256       +1     
- Misses      22686    22696      +10     
  Partials     1251     1251              

Flag	Coverage Δ
go	50.93% <9.09%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
...o/pkg/beam/runners/dataflow/dataflowlib/execute.go	0.00% <0.00%> (ø)
...ks/go/pkg/beam/runners/dataflow/dataflowlib/job.go	21.55% <ø> (ø)
sdks/go/pkg/beam/runners/dataflow/dataflow.go	58.65% <100.00%> (+0.23%)	⬆️

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[lostluck]

LGTM