[yaml] Add examples for Spanner IO in YAML

[reeba212]

Added examples with explanatory comments for reading from and writing to a spanner table

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[Polber]

Add a couple comments, but overall looks good! Thanks

[Polber]

@damccorm Are Spanner resources (specifically these) exposed to general public? i.e., would anyone be able to run this YAML as-is?

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[Polber]

Just one small comment, thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.31%. Comparing base (2573106) to head (92f8d24).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #32288      +/-   ##
============================================
- Coverage     57.32%   57.31%   -0.02%     
  Complexity     1474     1474              
============================================
  Files           965      965              
  Lines        153454   153454              
  Branches       1076     1076              
============================================
- Hits          87974    87956      -18     
- Misses        63276    63294      +18     
  Partials       2204     2204              

Flag	Coverage Δ
python	81.50% <ø> (-0.03%)	⬇️

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[tvalentyn]

R: @Polber

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[Polber]

Added a couple comments for fixing tests. I think other test failures are unrelated

[damccorm]

Thanks!