[BEAM-10941] Use standard sharding conventions for fileio writes.

[robertwb]

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Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark	Twister2
Go		---		---		---
Java
Python	](https://ci-beam.apache.org/job/beam_PostCommit_Python36/lastCompletedBuild/)			---		---
XLang		---		---		---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
Non-portable
Portable	---		---	---	---	---

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GitHub Actions Tests Status (on master branch)

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[codecov]

Codecov Report

Merging #12900 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #12900   +/-   ##
=======================================
  Coverage   82.34%   82.34%           
=======================================
  Files         452      452           
  Lines       54016    54018    +2     
=======================================
+ Hits        44481    44483    +2     
  Misses       9535     9535           

Impacted Files	Coverage Δ
sdks/python/apache_beam/io/fileio.py	95.84% <100.00%> (+1.77%)	⬆️
sdks/python/apache_beam/io/localfilesystem.py	90.90% <0.00%> (-0.76%)	⬇️
sdks/python/apache_beam/runners/common.py	88.75% <0.00%> (-0.45%)	⬇️
...hon/apache_beam/runners/worker/bundle_processor.py	94.45% <0.00%> (-0.14%)	⬇️

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[pabloem]

Thanks Robert!