exposing num threads as option

[joy91227]

Key parallelism is limited by NUM_THREADS which is hardcoded to 100 [1]. This could cause commit too large exceptions [2].

Exposing this option for users to tweak is a easy workaround before implementing a long-term solution that ensures that it never generates outputs that are too large.

[1]
https://github.com/GoogleCloudPlatform/DataflowTemplates/blob/main/v2/datastream-to-sql/src/main/java/com/google/cloud/teleport/v2/transforms/CreateDml.java#L44

[2] https://cloud.google.com/dataflow/docs/guides/common-errors#key-commit-too-large-exception.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 49.71%. Comparing base (04a7052) to head (9c6d307).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/cloud/teleport/v2/templates/DataStreamToSQL.java	0.00%	4 Missing ⚠️
...google/cloud/teleport/v2/transforms/CreateDml.java	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2349   +/-   ##
=========================================
  Coverage     49.71%   49.71%           
- Complexity     4714     4715    +1     
=========================================
  Files           928      928           
  Lines         56300    56305    +5     
  Branches       6083     6083           
=========================================
+ Hits          27987    27990    +3     
- Misses        26308    26310    +2     
  Partials       2005     2005           

Components	Coverage Δ
spanner-templates	70.38% <ø> (ø)
spanner-import-export	68.50% <ø> (ø)
spanner-live-forward-migration	78.36% <ø> (ø)
spanner-live-reverse-replication	80.38% <ø> (ø)
spanner-bulk-migration	87.74% <ø> (ø)

Files with missing lines	Coverage Δ
...e/cloud/teleport/v2/templates/DataStreamToSQL.java	4.47% <0.00%> (-0.21%)	⬇️
...google/cloud/teleport/v2/transforms/CreateDml.java	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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