Adding failure_mode parameter to Spanner Python transforms

[pabloem]

r: @johnjcasey ? : )

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (b6e19eb) 37.86% compared to head (2bf5002) 37.86%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
sdks/python/apache_beam/io/gcp/spanner.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #29529      +/-   ##
==========================================
- Coverage   37.86%   37.86%   -0.01%     
==========================================
  Files         690      690              
  Lines      101352   101356       +4     
==========================================
  Hits        38381    38381              
- Misses      61377    61381       +4     
  Partials     1594     1594              

Flag	Coverage Δ
python	29.00% <0.00%> (-0.01%)	⬇️

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R: @jrmccluskey for label python.
R: @kennknowles for label java.
R: @johnjcasey for label io.
R: @nielm for label spanner.

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[nielm]

Approved pending pre-commit checks passing.